BuddyPress.org

@arnonel, the parent template file is never accessed. The only possible performance hit you would see is with loading the additional CSS from your child theme. Since you’re styling it one way with one set of CSS rules, then overriding it with another, you’re basically doing the same thing twice.

It’s still the best/only way of doing this without totally writing your own CSS from scratch, or copying the bp-default CSS over totally, and never including it from the parent theme. If you go that route, you run the risk of not getting updates when CSS changes in the parent theme.