None of my engines have 15 inches of vacuum in the crankcase….or any as far as that goes. Are we measuring in the same spot? I usually tap the fuel pump block off plate or the valve covers.
most people think around 8, maybe 10 is ideal.
Get up around 15-20 you can be looking at premature piston pin wear.
Couple things to consider…. Pump is more help with a thinner ring pack, less help with a Jurassic era ring pack.
A non “ racy” oil pan deal is more likely to benefit from a vacuum pump. Fancy, well designed pan, less gain relatively speaking
Make sure the motor is sealed up good to maximize gains..
Most typical weekend racers can probably find other ways to make power, and often do. To me this is a race only deal and requires lots of attention to detail …