This is a belated post. It took me awhile to find my data. I have some readings from my '71 240z with original engine.
In 1981 with 90,000 miles my cylinders registered 165, 165, 170, 165,
175, 175. In 1984 with 122,000 miles they registered 135, 100, 135, 130, 145, 145.
These reading were from two different shops, but probably reflect actual softening in my engine during the period.
The engine now has 200,000 miles on it and will still accelerate smoothly (but not as quickly) up to the red line. It now burns a quart of oil about every 150 miles.