Yes nissan used upgraded internals on the S/C engine.

In early engines there was a recal that adressed the MAF sensor reading going over it's limit, and illuminating the SES and limiting the engine to 3500RPM. After that there is still a boost control electronic bypass controller that the ECU does control to limit boost to 5psi under certain circumstances. The bypass controller can be bypassed.

Yes the smaller pulley will boost sooner.

IIRC on my 02 Supercharged Xterra I would get the following readings:

Stock: 5psi@3000RPM, 8psi@5900RPM
2.3" Pullley: 9psi@3000RPM, 12PSI@5900RPM

The truck pulled 199RWHP on the dyno with the smaller pulley, K&N cold air intake, bypassed electronic bypass controller, and a free-flowing exhaust. That equals about 40hp over stock.

On a final note, I blew up the original engine at 70K miles, running 50K of that with the 2.3" pulley. After replacing the engine I didn't like it with the stock pulley, so I changed the pulley back to a 2.3", and proceeded to dyno it after break-in.

That's when I overheated the exhaust and killed the catalytic convertors which caused high backpressure and overheating of a piston (or two) and resulting in rod knock.