It's no big deal, no reason to focus on it.
If you are comparing your Z to your old Mustang, I would expect your Z to smash it by a ton. I have trouble really seeing that much power out of a NA four cylinder, but if it's got extra air, and I guess a lot of mods, I won't argue. I don't like to talk about numbers, myself, unless it's easily proven-track day on tape. Drag strip times, pr track times, tapes. Just because all of the variables. A few people have been getting pretty excited about A B testing, and like those variables, that is the other testing.
And also, suspension has a HUGE affect on performance. A close friend works at Roush, and they developed the new GT500. 670ish horsepower. The Roush Stage 3 Mustang has 540, but the 0-60 is still faster, and even the quarter mile. It's around 130 mph when the GT500 overtakes. These can't be published by the company as official results, since Roush develops it, But you can see it at the drag strip or track. Suspension wil do that.
I'd wait to do the sixth gear swap, until you have more power andless drag. The sweetest improvements are aero. You may need that power to move at that speed. Lowering my drag has really helped a lot.
Today I was getting 38.x when I approached the highway. I tested my o-60 time, and I got down to 31! I made 37.x finally, but still.
GT-R Versus 370Z - Car News - RoadandTrack.com
I can't believe the GT-R and 37/50 are being compared. I didn't think ANYTHING was the same. I mean, I doubt the Z is built in a hermetically sealed lab...or any of the countless other things they do for it. Now the new Zs are pretty sweet, with the stiff suspension, a better luxury feel, and improved power and aero. But most cars from 90-2010 seem to be half tried, in my view. Nothing from that time seemed really amazing. Cien, Sixteen, NSX, STi, XLR V, the introduction of the Escalade, and hybrids-concerning the US, that is.
But that's just my opinion.