I had L/C/R Sierra-1's and upgraded to Sierra Towers w/NRT. And I am 100% with Curtis and Merrymaid on this - the Towers really are a whole new ball park of game above the Sierra-1's. As Curtis said, that dedicated midrange makes all the difference, plus they punch deeper. After only a few moments of playing them, I immediately noticed the improvement and thought it was *well* worth the money invested.
I put the Sierra-1's on surround duty (7.1) and I eventually picked up the new monster Sierra Tower Center...which is amazing. I don't know how or why, but it is more cohesive and impacts cleaner than even the standard towers. Must be Dave's black magic.
So, I have exactly the system you describe (plus a pair of D-15SE subs)...my room is roughly 17x24ish (one corner is closed) and I sit about 12 ft from the speakers. I enjoy it so much I'm considering a RAAL upgrade
And tempted to upgrade the subs to F25's. What else to do with tax returns, right? In my opinion, it's not overkill. I could still use more headroom (heh).
Anyhow, where are you located? I'd be happy to demo the system if you are in my neighborhood (Nebraska).
Also, "kwarny" (nickname on this forum and AVS) came over to check out my system recently so you can ask him for thoughts - he's heard a lot of gear I think. Same goes for "tesseract67" on AVS, he had very positive things to say about my overall setup.
Edit - call me crazy, but I do love having the Sierra-1's as surrounds. It's a bit of a guilty pleasure, but between them and the monster STC, I *really* enjoy multichannel music...even just CD's hopped up on Dolby PL-IIx. I also think that Sierra-1's for surrounds are awesome with a medium (50-60hz) crossover for movies...get things a bit more punchy all around. I'm very happy I have it all set up that way.