This baseline would be much, much higher on a dynojet as numbers from a dynojet for two GT3's posted earlier this year show. A 997.2 GT3 RS with the same 3.8 liter although in a slightly higher state of tune put out 419.7 wheel horsepower while being rated only 15 horses higher than the 997.2 GT3 tested here which should illustrate how much lower this dyno reads.
Additionally, a dynojet run for a stock 997.2 GT3 is posted below with the car hitting 404 wheel horsepower much higher than the 329.4 wheel horsepower on the Gintani Dyno Dynamics. So, hopefully people understand they should pay attention to the delta and not the peak numbers as dynos can vary greatly.
997.2 GT3 Dynojet Baseline:
On 91 octane pump gas Gintani is able to extract another 12.4 wheel horsepower tune only which is a very nice result. That would likely put this motor with just a tune slightly above the GT3 RS 3.8 horsepower numbers. This would mean one could expect in the ~420 wheel horsepower range on a dynojet. Definitely impressive and one can see where Porsche extracted some power on the RS model, from a more aggressive tune.
Dyno video with the full graphs is below.