Lower rpms on the highway? Slightly less pickup, but would you really notice it in a Cobra? You'll probably put the power down a little easier if anything!
The viscous (R33) centres were described to me as operating more like an open wheeler when you're corning hard or with one wheel off the ground. These mechanical ones are meant to be better at transitioning torque from side to side and closer to a 1.5 way diff (semi-locking). Plus you get to say the word 'helical' a lot, which sounds really tricky to me.
Merv, even more tricky, check out these high capacity diff casings for when you go drifting in the Cobra!
http://www.driftshop.com.au/diff.htm
