your wrx already has limited slip rear and 5kg center lsd
one upgrade option would be 20kg ctr and a front helical lsd. prepare to apply Cubic Money. :wave:
this would give you true awd on gravel and snow...really wakes up the car grip-wise. youd probly want a dccd setup as well so you wouldn't have to deal with the full lockup at all times. in fact, in low speed hard corners, locked diffes make understeer worse, not betta. OTOH, a bit of rear diff bias puts the car in Mustang Mode so you can steer with teh throttle like ye olde days.
however, the stiffer diffs would suck on the street. lots of clunking/clonking as they do their thing, and a bit of herky jerky action at low speed like around parking lots etc. they'd also cost fuel mileage
probly the simplest/best diff upgrade for a street wrx is a factory sti 6-spd box with dccd. total subaru groovy goodness.