I searched this topic myself for a long time and finally bit the bullet and went with 225/50 15 Toyo Proxes R888r. I do not like the thread pattern on the Kuhmo all season tires so this was the only option at a decent price.
I don't think I will be taking this car to the track any time soon but I really like the look of this setup and I needed to get new tires when I bought the car. Instead of spending money for tires on the original Mag wheels I just got wheels and tires all at once. I know these tires will not last many miles, but I don't plan on daily driving this car.
I don't know why it was difficult to get examples of the fitment on 15x8 Rota RKRs being as it's such a popular setup for 240zs (S30s in general).
My car was lowered on Tokico 1050 springs (discontinued) I'm not sure what the drop is as it was done by previous owner.
I did have to roll the rear fenders a bit as there is a bit of rub when it goes over bumps.
Excuse the dirty car.
Ed.