Drop-a-Daimler

I have been trying to get some clean runs on new C55 with the G-TECH and can't get it to hook up for schit. I tried more gonzo runs last night, trying a real soft launch at idle, and still couldn't get the car to hook. My best run was a 4.8 0-to-60 with a 2.2 60' time (which sucks) and that was with no wide-open-throttle at all. In fact I have to let off the throttle when it brakes traction again near the top of 1st gear. I can't go WOT until well into 2nd and it must really be hurting the times. So I am thinking one quick fix might be to replace the tires. I hope to do a Quaife LSD soon afterwards too. I see no reason to do mods for power when it can't use much of what it is already making. So for now I can probably convince the OL that it needs tires. I would love to get a set of Michelin Pilot Sport Cup (DOT comps) for $280/ea. I was thinking Toyo Proxes and then Yokohama ADVAN then Kumho Ecsta (for the price). I know, I know: they wear fast, suck-*** in the rain, are like hockey pucks in the cold. Real problem is, I could not find a stock rear replacement size to match. The fronts are fine but out back all I could find is a 265/35/18 to replace the stock 245/35/18. It is almost an inch wider (which would be cool) and it also an inch larger in diameter. That means the speedo will read 2% too slow (58.6 instead of 60) which I am OK with. But the recommended rim width for the 265/35 is 9" to 10.5" rims and the stocker is only 8.5". What to do folks?