My 2 cents,
Owning a R33 Vspec and having 500+ runs on the Advan Performance 4wd dyno under my belt, we have noticed a few things.
1. Peter will only tune the GTR in RWD due to the attessa interfering with the amount of 4 wheel power delivery ( almost every run it's slightly different based on how the attessa feels at the time).
2. Based on the same tractive effort, the 4wd kw figure seems to vary by about 5-10kw on almost every run. This is also spaced days apart on the same tune so heat soak is not a factor here. The RWD figure on the same tune remained within 2-3kw every run.
3. My GTR's 4wd mode generally saps about 15kw from my peak power figure (within the variable bracket I stated at item 2.) It's simple physics as was said before, there is always going to be some power loss to the treads and more so when the engine is trying to drive 2 drive shafts and 4 axles instead of 1 shaft and 2 axles.
IMHO it's not the peak power you make in these things that's important, it's all about the torque you can produce to keep the 4wd system accelerating right up through the rev range which makes all the difference...
Fun too :-)