Having heard of but never listened to any of the components or speakers you list, I figure I'm qualified to be wrong as much as anybody.
As to the room size, I don't believe that is an issue at all unless it is deemed a very 'live' room acoustically. But then that room would be a problem for any speaker. And that's assuming that the wall behind the speakers is one of the short walls and there's not many windows, etc..
In fact, having had a listening room with your dimensions, I would go so far as to say that your room dimensions are near ideal for this size speaker assuming it is truly rectangular and closed off.
If you have the right amp for the 20T's I would speculate that the 20T's could be a real contender with the 5A's. If not, then I think the V5a's are probably the better and safer route. But then again, you're talking about $7k difference in price too.
-IMO