Not alot, I did try the Golf Recaros but no luck - you have to kill 4 seats to get two to fit, then weld, retrim etc etc, about £800 worth of work!
 
I was going to ask this too... do Mk1 Leon seats fit even? Maybe not as they are the same as the Mk4 Golf :(

I really like my OEM seats, the big problem I find though is that they don't go down low enough.