with the NA diff. That being said, if you are road course racing, then you aren't putting shock into the differential quite as bad as the way you do with fag racing.
I've never heard of an NA diff blowing out from road racing.
However, if you want the strength, and run a OEM NA gear ration (4.11), you could just swap in some mark leader gears. [ http://www.specialtyz.com/gears.htm ]