Personally I prefer twins on a GTR, all the stuff is there, just swap the turbos over. When going to a big single you need AFM adaptors, change the inlet pipework, exhaust manifold, dump pipe, wastegate etc etc. If you spent the same amount of money, the engine with the twins would make a higher average power every time.
I would go GTSS's 2530's for their response, 2530's for their all round ability or GTRS's if you really want that nothing...nothing.....hang on sensation.
cheers