Bought my 2011 XKR with 27000 miles on it. I'm guessing the seller (or maybe the dealer) used up the original rubber and threw on a set of new 19" Hankooks when it was traded. I have two questions:
1) I'd switch to 20" wheels if I thought they would make a huge performance difference and the ride didn't get appreciably stiffer (wife loves the ride quality now and couldn't care less about cornering). Will they?
2) With the current tires it is ridiculously easy to break the rear wheels loose every time I accelerate, especially when turning. Sure it's fun, but is a bit scary in tight situations. Would better rubber (like Michelin Pilot Super Sports) stick better? I had the car on a track once and the tires seemed fine during high-speed cornering - it's starting from a stop where all hell breaks loose, sometimes unintentionally. Or maybe I just need to get better controlling 505HP.

 

This has been covered many times in many ways on the forum so far, but I am quick to step in when someone mentions "performance" and "bigger rims" in the same conversation.
The two are in opposition to each other...... Smaller rims have far greater grip thanks to the flexible sidewalls, lower PSI, and larger contact patch.
Upgrade your tires to full on summer performance tire for the best traction for your dollar.

BOL
Vince

 

The grip may be marginally better, but turn-in with flexible sidewalls is marginally worse. A trade-off, for sure.

 

But we are talking traction here...

There are only two pluses to a big rim upgrade (turn-in and looks), and neither of those improve traction.

Meanwhile, the lower un-sprung weight, and significantly larger contact patch (10-20% more is hardly marginal) of a smaller rim win in all categories of grip from launch to full on cornering. The tires ability to deal with adverse road conditions dramatically improves as well.

Everything about traction comes down to the basics of how much tire is in constant contact with the ground. Given identical compounds, big rims loose every time.

Vince

 

The car is already stiff sprung, go for the best tires your wallet can afford on the 19's. 20's also wear your brakes and suspension quicker.