I look to the cobra replica builders who offer purpose built cobras for the track.
XCS 427 is arguably the most race ready replica you can buy, no expense spared for achieving the best handling possible, looks like 18" rims to me.
XCS Designs
JBL Motorsports is another one with cutting edge suspension and chassis designed for serious track competition, looks like maybe 17" rims there.
JBL Motorsports - Home Page
I don't doubt that the Avon CR6ZZ is probably the best rubber you can get for grip on a 15", I also don't doubt the engineers behind XCS and JBL picked 17-18 rims for a compelling reason.
Given equal drivers with comparable cars and rubber, I'll go with the larger wheels every time, look at the road course population across the board by class and see what they run, they all can't be wrong
We all get the vintage look argument, 15's are right for the cobra. But to spin your decision there into a case where you can also blast by the 17" guys in the corners is quite frankly laughable, not so if you are talking about equal drivers with similar cars and race rubber.
Joe