I'm curious if a slight diameter miss-match between the two rear tires might cause a very slight hands-off-the-wheel drift to the right.

Car: '06 Cabriolet with every possible JCW option.
Tires: Fronts are two brand new Toyo Proxes 4's. Rears are a nearly new Hankook Ventus V12 Evo on the left and a more worn V12 Evo on the right. Tread depth on the left is 9/32. 7.5/32 on the right.
Tire Size: 215/40-18 on all.
Pressure: 38 front, 36 rear.

I'm thinking it's not the rear as they're kinda just along for the ride, but I had to ask. It could be the fronts as I've had tires before that created just a bit of a drift and when I replaced them, the drift went away.

But......in this case, and in spite of my question, I believe the likeliest cause of the drift is because I knocked the suspension out of alignment on the right side. This past Friday, during some spirited driving up in Northern California (I recommend staying away from Matole Road and The Lost Coast), I hit a pot hole at speed and blew the two right side tires. No damage on the front rim. Slight cosmetic damage on the rear rim but the bead area is perfect.

So, I'm curious what y'all have to say. Cheers!

Oh, and of course, I'm going to get the alignment checked as well as the suspension components.

I think you need to match up all your tires - buy two more Proxes for the back, have it aligned and I bet you're good to go.

Since the backs are just spinning, I doubt that the difference in the amount of squirm from the extra rubber would cause too much of a dift, but I would get 2 more proxies, and then have the bushings checked for wear...then get an alighment. My car had a similar, very slight drift...and I had a worn bushing...fixed it, installing some new high performance ones, and then got an alighnment.
The mix of tires you are driving on is dangerous...sure 2 more tires is $$, but an accident when it rains or when you spin out on a curve will shirly cost more!!

Could be, despite same size all tires real size and overall diameter differ. Alignment should help. Check your lower front control arm bushings.

Well, I'm not that worried about having the Proxes on the front and the Hankooks on the back. And I'd really rather have the Hankooks all around. What I may do, since I'll be getting a refund on the Proxes I bought is buy another pair of Hankooks for the back to even them up, left and right. And with the Proxes on the front, I'll wear them out in short order and put Hankooks back on. (GREAT tire, by the way.)

Meanwhile, I'll be taking the car to BR Racing in Los Gatos for a complete check of the suspension front and rear. They're saying that the rear control arms on the Mini's are incredibly weak and if the tow driver hooked onto them to secure the car (which he did), they're likely bent. If the impact with the pot hole didn't already do that. They're recommending Alta adjustable control arms if they are damamged.

We'll see what happens next week.