Ditto on the 5.1 volts... You can still tune however, it's just difficult because once it hits 5.1v it's pretty much just a guessing game as far as tuning goes... With the powerfc's the stock afm isn't much good because you can't retard any more ignition off because there's no points available. So basically you just have to pick a conservative value for your ignition on load point 20 across the rev range so if the boost does spike, you won't melt a piston.
As mentioned above... Full throttle can, but part throttle would be very rich.
2535 is probably the biggest you'd want to go without getting a compressor/exhaust wheel mismatch... The 2540 is too small on the exhaust side, and take longer to come on boost than they should... They also seem to require a lot more boost to make decent power than the 2535 does. If you can find one, you'd be better off with a 2835R or a 3037... They're both really well matched turbo's, and should make power fairly easily.
When you say that the AFM maxed out, were yuo referring to the RB25 or Z32 item? I've got about 310 rwkw in mine (as seen by a number of forum members), and I think my max AFM voltage was 4.9v...
What sort of turbo setup do you have, and where aboust are you based?
Cheers,
matt