I worried about the same thing. I don't like huge or heavey bikes, but I swear the tbird feels lighter once moving even 2 MPH. At a standstill trying to push it, yeah, it feels heavier. but because they set the center of gravity so low, the bike handles way better than the america or speedmaster. I had the latter, tho same bike as the america.
However, and some will disagree such as my pal OGB above, but i wouldn't necassarily do it just for the power unless you go with the 1700, either by buying the storm or the regular Tbird and having the 1700 kit installed. (which is what i did) I just don't feel the 1600 has enough more balls to make it worth the extra $. It DOES have other things....better handling, much better build quality and lots of little features you don't notice till you get one. But IMO, and it IS just my opinion so take that for what it's worth, i just wouldn't pay all that extra initial maintenance costs for the 1600's performance. To me the 1700 makes the power feel like you expect a bike that size to have, but i don't feel the 1600 does. With TORs it's not so bad, they really make a good bit of difference. (the short ones do at least)
Anyways, thats what i feel after going from the 1600 to the 1700 engine. By the way, the majority of the power difference is due to the 1700 having hotter cams, the extra 100cc probably contributes 20% of it i imagine.
So in a nutshell, it's truly a better bike in every way, build quality, features, handling, cruises at much higher speeds with ease, etc. But whether it's worth it to upgrade to the 1600 i can't say. For me MAYBE with TORs it's enough to have made me happy i did it. I'm not sure at this point because i'm blinded by the power of the 1700 with TORs. It's far far more powerful than my speedmaster with pipes and mods was. It's an expensive upgrade both initially and maintenance wise, so weigh all your needs and desires against the cost very carefully.