I dont know, if you think about it bumping up to a 2.5" catless exhaust system would probaly have less backpressure than the stock 2.25" with 4 cats (or 6 if your NPI) and no mufflers.
Good question though. Mine didnt really feel like a slouch or anything without them, but the X pipe made it run pretty nice up top.
Who knows...
Well, the mufflers add restriction, no matter what brand or design they are. Even a LM2 adds some restriction.
I think no muffler at all isn't a good idea personally.