I can say the tach was probably off by 150-200 RPM low (they always are...sent my Autometer dual range tach because it was acting wonky and it was off by 350 RPM at 7500 which meant it was really 7150 RPM) and there is a mile of difference between 7500 and 8000 RPM.
And there is a big difference just getting enough spring to make 8000 RPM. It another deal to make power up there.
Edit: forgot to mention the Autogage tach I replaced with the dual range tach was off by about 125 RPM at 1000 RPM. I could check that with my ignition box.
Most tachs are off.