Prestiges on OP Prison I don't think would work, it'd change everything to the extreme. Ares has already been said to be released, with more mines. Hopefully we'll be seeing change in the near future. Baring in mind, OP Prison just turned a year old on the 11th (2 days ago).
I think there should be like 1 rank after Z that would be pretty hard to get to... It's already hard enough to get to Z, no support for the idea of 10 prestiges.
People will just grind at Z, pay and alt, and keep Prestiging, there is no true way to prevent abuse as such That's why the idea of having ranks after Z is a better plan, as that to relates to normal prison, OPPrison's similar counterpart. (Ranks such as Free, Citizen, etc.)