I have a largeish room 350sq ft with 9ft ceilings, and run 340 front three and 200se surrounds, it sounds great.. I would go 200s for the surrounds, the 170 {I have tried them as surrounds, as I own almost every ascend speaker made , just not the sierra towers, yet} and noticed ZERO difference on surround duty, sure if they were in the front the 170s are much stronger, but for surrounds the 200s will serve you very well... Plus keep in mind the 170s are a pretty big bookshelf speaker...
The 340s will have no trouble, and I have ran the 170s in the front of my room with a 340 center and been impressed, IMO the only benefit of going 340 for the front left and right is if you are going to use them for music... My room isn't small and I used the 170s up front with a 340 center for a couple months, and it sounded great... But since I have the 340s they made their way back when my other speakers came in...
I play with my speakers A LOT, I move them around and check what sounds good with what... I actually own a evo2 center that is in the box still because I wanted to see if I liked them in my theater better than the 340s, I didnt, but now Im stuck with a $600 evo2 center that I have no use for, lol...
If your budget is $1600 just for the 5 channels and not the sub, I would go Sierra 1's for the front 3 and 200se's for the surrounds that comes to $1568 shipped and you will NEVER have to upgrade...
I own a pair of Sierra 1s with a pair of mono blocks for music and they sound awesome, while I like the 340s in my theater because the center fits over the fireplace better {and I dont own a sierra 1 center}, but if I ever switched from the 340 front end, I would get the sierra 1's...
The ascend speakers are just realistic, there is no other way to really explain it, they color NOTHING, they just reproduce what is given to them, no strong highs, or strong mids, just how its given to the speaker it plays the material... I really like them...
Also I have tested many speakers, and for theater I would always go bookshelfs, I have tried a lot of towers, and prefer a strong bookshelf upfront...