After reading and hearing a lot of stories about the handling of these bike engined minis myself, I was also a little concerned and wondering whether I had done the right thing buying the Z cars kit.
However, after driving the car around Curborough, I was pleasantly surprised that the only problem seemed to be getting the power down coming out of the tighter bends which was put down to the tyres that i had fitted had gone off. Handling wise it seemed pretty good. I also have a lotus elise which i have driven on a few circuits, so I know what a proper handling RWD car should be like..
I remember thinking at Curborough that the handling felt almost like the lotus with the exception of having to put a lot more power down through skinnier, hard tyres. The power to weight ratio is ~ 350bhp/ton vs 165bhp/ton so quite a difference.
I have quite a good smooth test road near where I live which is a very fast sweeping right hander. In the lotus in the dry i can get about 95mph going around the bend. In the R1 mini i could get about 85mph before having Z cars set up the suspension. After setting up i can get 90mph easily and maybe 95mph if i am feeling brave! Also, after the setup the lightness in the steering has disappeared which was another one of my concerns about lack of steering feel. That has gone now and the steering feels very good. I think with my new Yoko A048's there should be even more improvement.
So, overall, if you get a decent set of tyres and a proper suspension setup then the handling should be very good.
PS When did you go round Donnington?? I saw Lee Oliver at Cadwell and he said the car was handling very well...