Chris ran 1.21's around sandown with RE55's on a stock RB26 engine/sump.... 250rwkw.... no issues with spun bearings, so there was no Benno Trickery there.
I've run 1.22's around sandown with RE55's and a stock sump (as far as i know anyway) But there is other Racepace trickery with the oil in my engine, but i think the sump is still stock. I don't think the sump is the issue, but gtst could be different to GTR... It wouldn't hurt to put a baffled sump in, but don't assume its needed....
I'm with Troy... Penrite 15w? I don't know if it was the oil, but i defiantly recommend the Castrol Edge Formula R 10w-60 stuff... that's what most of us run....