Ok, first off, the S2000's little 4 banger puts out about 240bhp, and the car is about 600lbs lighter then the 370z. That motor is putting out a 120bhp per liter in comparison to ours which is about 89bhp per liter.
Real world test on the S2000 is 0-60 5.7 average, 13.8 1/4 mile. Now, compare that to the Z's numbers and tell me how much do you really think you were going to pull on it?
Also, your assuming they were stock, how do you really know, could of had a mellow exhaust and a tune, who knows, but even a stock S2000 will give you a run for the money. Heck my Mini Cooper Turbo sounds pretty stock, but with the tune I have and the mild cat back, intake, etc. it has surprised more then a few Vette's, Mustangs, etc. putting out 200fwhp and it's only 2500lbs :-)
Damm near as quick as the 370z.
Anyway, don't feel bad, there is nothing wrong with your car, you just didn't know what you were up against
On a side note, my Z, without a doubt, runs a little slugish when the oil temp hits 220, I can definitely notice the difference in responsiveness on a hot day compared to when the temps are in the 180 range. Only fix for that is an aftermarket oil cooler, which is something I will definitely be purchasing in the future.