Yes. I couldn't believe it, based on the performance of the two cars, so I did some research.
I found two quotes from books written about the Z:
"The four speed automatic was also available in the Turbo but in deference to the auto boxes longevity, the engines power output was cut back to 280bhp by different camshafts and fuel injection maps. Torque remained the same." From: Illustrated DATSUN/DATSUN Sports Car Buyers Guide by John Matras.
"For various reasons the 300zx Turbo in some versions is rated at
280bhp. This applies to the Japanese market, where the manufacturers
have an agreement to limit outputs to 280, to automatic trans
models in the US, which have different camshaft profiles and fuel
injection settings, and to the European market 2+2 because of its
setting for lower-octane unleaded fuel." From Nissan 300ZX - The
Enthusiasts Companion by Ray Hutton
I asked people I know at Nissan USA and Nissan Japan to confirm the horsepower rating of TT's with automatic transmissions.
The response I got from a contact at Nissan USA was:
"Aside from minor changes in the ECU, the cam profiles & turbos
are also different between the M/T & A/T models. The A/T has 240 degree
intake cams & 248 degree exhaust cams (the M/T has 248 degree cams for both
intake & exhaust). Also, the A/T uses smaller turbos for more responsive low speed
acceleration (the A/R ratio is 0.63 for the M/T & only 0.54 for the A/T). Even with 20HP less, the A/T turbo is capable of beating the M/T turbo to 100MPH."
I checked the fiche in the 'DOCS' section:
There were always different ECU's for manual and auto cars.
The camshafts were always different on the autos until the 1996 model year, and the part numbers changed three times. In 1996, Nissan discontinued the variable valve timing system, and rated the TT engine at 280 HP.
The exhaust manifolds (from the cylinders to the turbos) are the same for all years.
The turbos were always different (see Nissan USA comment above). They went through two part number changes on the right hand one, and three on the left hand one.
The exhaust housings (which attach the pre-cats to the turbos) were different for manuals and autos only from 9/93 to 9/94.
The manual and auto had different pre-cats, and the part numbers changed three times from 1989 through 1995. Both cars used the same pre-cat for the 1996 model year.
The H-pipe was the same for both manual and auto cars throughout production.
The main mufflers were different throughout production - even for the 1996 model year.
The auto has a different radiator, to accommodate the need to cool the transmission fluid. There were no part number changes throughout production.
From all of the differences in components, it's pretty clear that Nissan did some serious engineering to get the two cars to perform much the same way even though the auto model is down 20 HP.