There is no end to the route planning advice available. Myself, I would say there is plenty of great riding in CA, NV, UT, NM, CO, etc to keep you busy for years and years...but if you want to visit the flat states like TX, etc then that's up to you. I will say TX does seem to know a thing or two about good food, and I don't even care about food, so that's saying something.
As for the bike itself, I just did 3500 miles in about 10 days on my S3 last month for the TnT run. I avoided interstate 99% of the time. My longest days were around 575; my shortest only a couple hundred. Here's what I would suggest:
I too have dual high-mounts, so saddle bags were a no-go. I used a small (10 liter?) tank bag (Marsee Corona mount - they don't offer it for Triumph any more sadly) with a quick-release so I could easily fuel. I also had a small Coretech tailbag (
http://www.revzilla.com/product/cortech-sport-tail-bag-2). Since the rear seat on the S3 is so small, I made a flat plate that I attached with a couple of straps around the seat that gave a larger base for the bag to sit on. Basically a home-made version of this:
http://www.thecycleguys.com/fast-rack-luggage-rack/ Mine was heavier as it was steel, but it also cost me less than half to make it instead of buying, and mine was more secure as I had more than just velcro holding it on.
Here's a pic of the whole rig:
The tail bag could have been smaller, but I hauled an iPad and a pair of shoes to wear when off the bike. Those took up a fair bit of space.
I find I pretty much can't use the map pocket on my tank bag with this bike - it's located too far back and I can't look down enough to see it easily. I already had a Garmin Montana GPS that I use for hiking, so I bought a RAM mount that attaches to the clutch mount and supports their AMPS powered mount. It looks a little goofy in non-touring mode, but it puts the GPS up nicely in your line of sight. I then added Garmin's City Navigator NT software and was good to go. Also, fwiw, the Montana's touch screen worked fine with my gloves on.
If you stick to the slabs (but why would you???) you'll likely be fine, but assuming you take the 2-lane highways you could find crossing NV might be a fuel-challenge. I had a little fuel problem coming out of Tonapha when I saw a sign saying next gas 163 miles...after I'd already covered 10 or so miles to get to it. Do the math: A Speed Triple on near-deserted highways where you could easily run 100 mph for hours...means I couldn't make that jump! I ended up going back to the station closest to that edge of town and picking up a 1-gallon washer fluid bottle, filling it with fuel, then stopping about 50 miles into that leg of my trip to add that fuel to my tank as I wasn't real comfortable with it riding around behind me.
A safer solution would be something like an MSR fuel can (take a look at REI or someplace similar) or some other certified container you could put a little extra gas in. You don't need a full gallon, but half a gallon might be the difference between a lonely walk and coasting into the next gas station. That run ended up only being a little over 160 miles, but all the same unless I really baby the bike (not likely!) I'm usually seeing the reserve light around 130 miles.
Another thing I ran into that you almost certainly will hit as well - tire usage. I had to pick up a new rear tire along the way. Most of us are running sport tires on an S3 - they aren't going to hold up that well to thousands of miles of highway riding. I'd consider switching to a good dual-compound sport touring tire like maybe the Pilot Road III. Otherwise you're simply going to square off any sport tire and need to buy something half-way through the trip...and you'll likely pay a lot more on the road than if you do it before hand.
The only comfort item I changed on my bike was the grips. My throttle hand in particular gets really sore/cramped up after a few hours. I added a set of grip puppies from CA Sport Touring. (
http://www.casporttouring.com/cst/motorcycle/GRIPPUP/GPSMALL.html) They're just up in Hercules, so you could easily stop in to pick up a pair. At $10 (installed no less...the owner just walked out and put them on my bike in the parking lot while I was there) if you don't like them you can just cut them off and keep going. They slip over your stock grips, so if you can remove them and just keep going. They did help my hands on the ride, but it took a long time before the soapy water used as lube to get them on finally worked it's way out. Until then, the grips wanted to slip around a bit which was very uncomfortable on any sort of aggressive riding. Since I run bar-end mirrors, I wasn't worried about the grips coming off, but it was still annoying.
A throttle lock of some kind would be a worthwhile item to look into as well for long highway stints. I've never actually bought one, but every time I do a long piece of highway I know how much I'd like to be able to shake out my throttle hand from time to time.
I'm about 5'10", 170 lbs, 32" inseam and have no problems with the stock seat. It's basically invisible - I never even gave it a thought. Occasionally my knees get a little crampy after several hours, but just moving my foot around to different positions on the pegs is usually enough for that. On the other hand, the base of my neck does get stiff after a full day of riding. I have no answer to that other than ibuprophen. Throw a bottle in your tank bag; you'll thank me at some point, I'm sure.
Riding gear wise, you don't need to run out and buy a full Aerostich suite. Nice, yes, but you can probably get by with what you have. I wore a RevIt Sand jacket (
http://www.revzilla.com/motorcycle/revit-sand-jacket) with a dual liner system (rain/wind and thermal) but only took the rain liner. I did take a fleece that I wore under the jacket when it was cold, but chose that instead of the jacket's dedicated thermal liner since a-I wasn't in really cold weather (temps ran from about 50-100 for me) and b-I could wear it off the bike so it was more versatile. I wore my standard Dainese sport gloves and boots, and a pair of A* overpants that are decidedly NOT waterproof. I did bring a ligthweight pair of shell pants I take hiking that I pulled on over the A*'s when I had to spend a full day in the rain, otherwise I just dealt with getting wet if it was only a small shower. The jacket, btw, did fine on rain. The coretech bag not so much, but I also admit I didn't stop right away to put the rain cover on it.
Most of the rest of my clothing was the same stuff I take hiking - mostly because it is wicking, fast drying, and rolls up very compact. Much better than a cotton T-shirt and jeans most of the time. A trip to REI will also show lots of options in this department, but that stuff isn't cheap and I wouldn't run out and buy it just for one trip. I had all of it already as I enjoy hiking, snowshowing, backpacking, etc.
Here's the whole story of the trip:
http://www.triumphrat.net/ride-trip-reports/218604-to-taos.html