The PCA rules say:
"Engines with the 9.5 compression ratio may use a two degree offset camshaft key."
The NASA rules say:
12.5.5 A camshaft key offset 2 degrees advanced is allowed for motors equipped with the low compression 9.5:1 pistons.
ARG!!!! You can do all of those with the 9.5 pistons a favor by turning that into a 2 degree retard and retesting your car.
I doubt that will get you the 10 ft*lbs of torque you are missing near redline, but it will be a start as it seems to me that you have a flow problem. It's been years since I fiddled with the Porsche AFM, but I have modified over 100 Spec Miata AFMs.
You have already relaxed the spring tension in your AFM, but you can do more. Watch the wiper arm while on the dyno and note the position of the arm at redline compared to the position of the arm with the flapper door in the fully open position. You want the flapper door to get as close to fully open as possible at redline. This means less spring tension (CCW on the spring) with a wiper arm adjustment (CW) to counteract. This is not so easy unless you make up a little electronic helper with a second AFM connector, multi-meter and DC supply voltage (12 volts on the early AFMs and 5 volts on the later AFMs). With this "tool" you can easily tune the AFM with the car on the dyno and can them make small adjustments for altitude, air density, etc. at the track. I adjust based on the output voltage with the flapper in the fully open position.
Note that if you go this far with your tuning, you would be well served to run a wideband O2 sensor in your car at all times.
If you can't get your high end numbers up with both of those tuning tricks, then I am sorry to say that your flow problem is a bit deeper. Bad casting, bad valve job (looks like it's sealing, but not flowing), etc.
PLEASE PLEASE PLEASE....do the offset key first without anything else and report back those results.