I have just got my RB26 260Z running. I have a 3.9 diff I think, I have an Apexi D Jetro, but not yet dynoed or properly tuned. I have the RB25 GTS 5 speed, and whilst the motor is forged everything, it is standard, other than bigger turbos. So far I have found that the gearing is far too tall, and I will be changing to a 4.1 as well. I also have an R32 GTR (390hp at rear wheels - cam & apexi & N1 Turbos) and the gearing is far lower (4.1 diff) and the car is much better, and the gearing not too low.
Once I am run in and I have fitted the 4.1 & Quaiffe LSD I will tell you how it goes, but I agree with needing the 4.1
regards Mike