I think that a 350-400 cc TW type motorcycle would be awesome. Not so much for trails or FS roads like I typically ride, but for the road. I took a 80 mile road trip the other day and for the first time noticed my TW's low power. Riding at 55 (60 indicated) was no problem, unless I hit a headwind (which I did) or a hill (many) or both (most of the ride).
But then I wonder if just about everyone wishes their bike had just a little more. Is there a real Dual Sport out there that people who buy it don't start modifying for more power or wishing they could?
On edit: I got so wrapped up in my own thoughts (thread jack) that I didn't write what I meant to when I first read your post. On the DR, what would you think about installing a smaller front sprocket or larger rear sprocket to gain a little more low end (at the price of losing some top end of course)?
As for other options, a 225 and 250 TW has been done. That gives you the TW with a bit more power.