I use 555 in electric and nitro. It is what is recommended, a good starting point, and it works for me. You can go down to 3k or 2k in the rear if you don't think it rotates fast enough for you with 5k in it.

Speaking of diff. gaskets, I use the Thunder Tiger ones. They come in a pack of 10, and have 3 different thicknesses available for the same price of 4 from AE.
I have found the thin ones seem to be the closest to the stock AE gaskets.

http://www3.towerhobbies.com/cgi-bin...?&I=LXAKBL&P=7

http://www3.towerhobbies.com/cgi-bin...?&I=LXAKBK&P=7

http://www3.towerhobbies.com/cgi-bin...?&I=LXAKBM&P=7

Stephen