My brother errs on the side of caution and gives everyone the same amount of Karma, regardless of individual player performance. When my character brought in her cop contact as extra runner on a rescue-someone's-kidnapped-daughter run, even agreeing to cover part of his share if we didn't manage to recover the ransom money (the client didn't have enough spare money for a full extra runner), the cop ended up being crucial in keeping the funds from disappearing, resulting in a nice bonus for the team: when I pointed out how fortuitous my decision had been for the team, we
all got +1 Karma.

Anyway, hypothetically speaking, 15-ish karma can be a pretty big difference for magic users and technomancers, since it covers the cost of a single initiation/submersion, but other than that, it takes a lot of karma to get more than one extra die compared to someone else, especially if you only look at the top skills. So as long as someone doesn't consistently get 2+ Karma more (or less) than the others, I don't think it'll really be unbalancing.