A worn motor mount of the headers rubbing can be picked up as knock.
Did you have to bash the firewall to make the o2 sensor clear? That was a common rub spot with those headers.
I do want to get a few facts in though. Per the high octane timing curve for a 2001 GTP file, the timing is as follows. ( I'm too lazy to screen shot this).
In boost the cylinder airmass is going to at or above .80 to .90g just about all the time. With that said the timing from say 3,000rpm to 6,000rpm goes from 13 degrees down to 9 degrees. Timing slowly drops away as rpm rises.
The computer also use a timing adder based on the commanded air fuel ratio. That value is 3 degrees at 11.7 AFR and 6 degrees at 10.7 AFR. So range of timing can vary from 9 to 19 degrees. Then you have intake air temps that could reduce the timing. Anything over 104 degrees remove 1 degree and once it's over 120 degrees it removes 2 degrees. So if it's really hot outside or the intake is pulling in really hot air it can reduce the timing.
So timing can vary a little bit but a constant 9 degrees mean something, likely knock retard is pulling probably 5-6 degrees away.
