Steve, I would say that the fact I couldn't stop wheelspinning was more down to my lack of driving skills more that RLTC cutting the power. - I just couldn't bring myself to back off the throttle!
I've played around with couple of non-RLTC equipped Supra's in dodgy weather conditions and without any shadow of doubt, I had a noticable edge.
When I went to Elvington, the RLTC only cut in in 1st and a second or so in 2nd - And it was pi$$ing down that day!
Santa Pod was exceptional. I've never driven on anything so slippery at that speed.
I did turn the TC off once... Only once 'cause it scared the crap out of me! I was counter-steering ALL the way up the strip. It really felt like I was on an ice rink! - Perhaps my F1's need replacing, they're about halfway down now.
I'm more than happy to meet up with you and give you a demo - You can have a drive and see how you get on with it.
As for tyres, this time of year, with greasy cold roads, you'd be lucky to get much in the way of grip at the best of times. I do believe that the more rubber on the road, the better.
I'm running 275/35/18 on the rear and 245/40/18 on the front.
I'd think that if you are putting bigger rears on, you want to increase the fronts by the same amount. - I've heard it can upset the balance, (Read as, I have no Idea, but that's what I've picked up from reading previous posts )
Did that make sense
edited to add, with RLTC off, that was my slowest time. 15.946