Is what on a YB?

Aside from the ex housing options, GT42s have 2 compressor options which is what changes the ratings- a 850bhp version, and a 1000bhp version.

For T66s and T70s, well it depends how fancy and efficient they are for their size, i know of certain "T70"s that have been proven at 930bhp (no nitrous).

When it comes down to it, all these T66, T70, T76, etc etc are all "T4 series" based turbos (generally T4S or T4H) just with various wheels, housings, etc etc to make them into whatever.

Even HKS call one of their turbos the T4Z and thats rated to over 700 (Again, a Garrett T4 based turbo)