I replaced mine recently with 6x9 speakers but had to make some adapters for them to fit in the original positions.From what I can see you will struggle to get anything bigger than 4" square speakers to fit with out some sort of modification.
I did fit some JBL 4" speakers in the front at the same time and the sound quality is really good
Im sure you can find some really expensive 4x10's but the fronts I have seen drowns out the backs. unless you want to fade the fronts down and have them equal, but it will be very quite sound. I need to get some new back speakers.
Ive got some really nice boston acoustic speakers in the front, and they do the job nicely. I kinda like the sound up top, as apposed to in the doors because it sounds more acurate to me. I need to get some new back speakers too. will check out those autobahn's you speak of
I recently got a pair of Pioneer 4x10's, 35wrms for $35 (inc. shipping). I'm partial to Pioneer I suppose, my other car is all Pioneer (for now...might not be once I finially switch full-time...).
Been too cold since I got 'em and I've had other things to deal with so I dunno how they sound. Wish I could easially have gotten 1 of the pairs of 6x9's I had in there, looked like to much work to me.