Tommy,
The less pressure you have in the crankcase the lower the ring tension you can run = more power.
My race motors are built to pull vacuum based on the ring package. The motor is sealed air-tight, using double-lipped seals for this application and a 5-stage drysump pump provides the negative pressure. The ring package in these motors use low tension .043 second and a 3mm
oil rings and MUST have negative pressure to seal properly. There was a 14hp gain on a 775hp motor from the neg. pressure and the low tension rings.
My Cobra motor is built very much the same as the race motors but with rings suited for a street application. Without the ring package I didn't feel pulling vacuum was warranted, especially considering the small gain percentage-wise.