Every dyno is different but your numbers do seem high with your mods and usually mustang dynos read lower. Are they SAE corrected? Post the graph.
I've had a real long conversation with a dyno owner who stated that by adjusting some variables in the program, he could make the car show much more horsepower than it actually had. A lot of shops will tweak the numbers a little for "bragging rights" of the dyno queens.
My car is an M6 with less drive train loss and comparable mods and a full tune. I'm only putting down 350/350 to the wheels.
Keep in mind because every dyno is different, it should only be used for reference of improvements made by adding mods. The true test is at the track. If you indeed have the HP claimed, you should/could be in the high 12's on a warm night with a 2.0x 60'
The auto GTO's I've seen with your mods have run low 13's. The last time I went to the track, it was in the 80's and I was running 13.2x and a bone stock auto GTO was running 13.4x Both of us being very consistent throughout the night.
GTO vs. GTO - Video
