mixing and mastering are two completely different things.
mixing is getting the relative levels of the parts in the right balance (which is a highly subjective thing usually decided by the producer and the artist).
mastering is about dynamics, optimising the sound, making it more punchy, warm, and full, while raising the overall level (volume) and highlighting details that aren't already apparent.
Brian Gardner at Bernie Grundman is a mastering don. It doesn't get much better than that.
I would say a lot of the problem with SA hip hop is the mixing and EQing, not the mastering. Not many of us are working in multi-million dollar studios either. Having said that, I'm happy with Planetary a**ault.
I'm going to take the track I just did with Wordsworth to a specialist mastering studio to see if I can squeeze out some extra quality.