I also have a R33 GTR V-Spec and I found that induction and exhaust mods made a considerable difference.
My car is stock apart from Apexi pods in an air box, adjustable cam gears, Power FC, highflow cat, 3.5 inch HKS exhaust and 14 psi boost. On a conservative tune I am making 250 kw at the rears at 8,000 but it is making just under 240 kw at 6,000 and then slowly climbing to 250.
This results in a very driveable street car with strong induction noise and a deep exhaust note.
When you dyno a R33 GTR the normal practice is just to remove the front drive shaft.