Bayhawk, you are quite right. Not all.
More than from a random selection of similarly bred mares, assuming nearly half end up tried as performers.
If NO ONE has performing mares, then there are none to choose from and your choice must be based on bloodlines alone.
But just as you are statistically making a better choice by breeding with a ‘proven good’ broodmare than a ‘proven poor’ broodmare; you are statistically more likely to breed a better candidate from a top performance mare than any other class, including unraced.
Note this is not the same as a top $ selling prospect.