Originally Posted by wilsonp

The ratios and final drive ratio for auto and manual are entirely different, which may explain some of the difference.

I don't believe dyno tests show a significant difference between the two - 370z.com Magazine shows 7AT stock at 267hp / 220 ft-lb versus a 6MT stock at 262hp / 210 ft-lb (different days, same dyno), so the 7AT really gives up nothing at the wheels either.