I don't thknk you mean differential. I think you mean the rign and pinion gears...i.e. axle ratio. The differential is what splits the torque between the wheels. More gear would improve low end response. I don't know if these cars use the R200 rear assembly from the Skyline and 300ZX, but Nismo does make gear for those. Your problem though may not be what you think. It may be something I noticed when I first drove one. The traction control limits the power in 1st. As soon as I shift into 2nd, it feels a lot stronger. Turn of the traction control and 1st is a lot more spiriting. BTW, I had the exact saem thing when I was test driving Z06's. With the traction control on they would feel slow in 1st. When I would shift to 2nd, they would want to immediately break the tires loose.

04' G35 Coupe(on order)
6-sp Diamond Graphite/Willow w/Premium