#1 The turbo makes the power
#2 The supporting modifications make it work and survive
Turbochargers
Do those two statements above make sense to you? The engine itself is just a big air pump for the turbocharger. We have a hard time changing the engine characteristics without getting into the RB26 internals. The more air the engine can push at the exhaust wheel, the more air the compressor side can ingest.
The bigger the inlet, the bigger the compressor wheel, the more air it will flow at a given RPM. The more it weighs, the harder it is to get it up to a certain RPM. Turbo sizing is a bit of an art and a science. You have to read into some of the compromise to achieve certain levels of performance. If you aren't sure, then you go off past data. If you want to experiment, first check and see if anyone else has done it. The current world in turbos for GT-R's, the popular route is a single turbo, from the OEM twins. While we personally prefer the OEM look of under manifold twins, that look isn't for everyone.