Blowby can rob performance for sure... dependant on how effective crankcase ventilation is managed... scavaging the crankcase; putting the crankcase under a vacuum is the best solution for best performance... I remember several occasions where the valve covers blew off, and crank seals blown out due to excessive blowby, just from one cylinder top ring failure.

I've seen some badly scored cylinders with poor compression leak down tests, but when running seal with very little blowby, and they have been running that way for many miles.