Atlas 1.5 Diff/Tarmac spec mk2 escort questions

[Zico]

New build and 1st test day at Kames.

Its fully seam welded, weld in cage, 5 linked, fully floating atlas, 1.5 diff, coilovers all round with a 2.3 JRE XE on Jenveys putting out 297bhp@8krpm, The engine has been mounted 17cm further back for improved PMI, it has15" superlights to allow for the bigger AP racing 4 pots and is on soft intermediates (iirc?).

Its our first tarmac spec Escort, the T5 big tooth gearbox is proving problematic, slow shifts/missed gears etc and just getting used to the high grip levels and the set up.

Unfortunately I dont have the spring rates/suspension travel figures at hand. At 1:56 here- http://www.youtube.com/watch?v=vP3RRSyMJC4 on the long left hander the inside rear while cocks but instead of the diff locking, the wheel is just spinning in the air then when it comes back down to make contact the car will go a little sideways on the exit of the corner... should it require a decent amount of torque differential on both wheels to lock the plates ie; is it more likely to be a spring rate related tyre contact cornering issue... or are the plates simply more likely to be worn and need replacing ?