I, too, want a different seat for the Speed Twin. After an hour, my butt gets numb on the stock setup. I've had 4 different Corbin saddles for various sport and sport touring bikes over the years, and that's what I'm upgrading to this year. Yes, they're firm. Yes, they have a break-in period. But the firmness is very different than the stock plank, and the contours fit me really well. More importantly, I've always been able to ride for hours on Corbin saddles (Gunfighter and Gunfighter and Lady models - terrible names ....).
I used Sargent Cycle years ago to redo the saddle on my wife's old Honda Shadow and I was really, really impressed with their work. The saddle they built for her lasted many, very comfortable years. But they require using your original pan, and I'm not willing to do that with the Triumph. Maybe if I pick up a pan on eBay? Last I checked, Speed Twins weren't a pre-made option for Sargent, so they'd rely on your own careful measurements and input to build something truly custom. That worked out for us on the Honda but, again, not sure I want to take the chance when Corbin is a known good fit for me.