The Auto Diff is geared down alot lower than a 6sp diff as you have said so with the GT35 i feel it will be ideal for 95% of the tracks in Europe. I feel no need to have a high top speed when i doubt the car will be getting anywhere near 170+ on any European tracks. With it being also 7kg lighter it does win it for me.
Get a ride in Pigs car with his T61 and a Auto Diff.. It produces the highest tractive effort i have seen yet at SRR with a Supra.
Steve@autotec has built this diff and have been on his 700+ Bhp Supra for about 20,000 miles now. The launch control of the Solaris has also been demonstrated many times with it on also
Not planing on Diff cooler at present but will be another thing on the R&D list