You will need modifications for a 255/25/21 to fit on a 21x12.5

I have tried many combos....21x11.5 , 21x12, 21x12.5, and for tires I have tried 325/25/21, 325/30/21, and 355/25/21 (Pirreli) ( the 325/25 vs 355/25 is big difference. )

If you do not want the stretch look on a 325/25/21 (21x12.5), but still want to use 21' you can run 21x11.5 or 21x12 to minimize the stretch with a 325/25/21.
I also did not like the stretch on my 12.5 with 325/25/21 and if you run 21 inch tires, the selection for us is limited compared to 20's

Having 21' in the rear is nice but tire selection sucks and you do notice the handling is not as great with 21 (even if the wheels are lighter or close in weight to oem).

You have more tire selection with 20 in the rear and its safer for fitment.

I have a set of 19/20 and 20/20 currently on order and will mess around with different combinations. will keep everyone updated