I agree that Mix is better in those spots, but don't we have DPs playing those two positions? (I always get confused about what is a 6 or an 8).
I'm no expert so I could be wrong but I always think of an 8 as a box to box mid and 6 as a CDM holding mid. Lampard was an 8 for his whole career but doesn't really go box to box all that much any more, he stays more forward than a typical 8 at this point. Pirlo is technically a 6 but he doesn't play much D for a 6 so we play Jacobson next to him as another "6" to help on D. For Mix to play a "6" he would need to take Jacobson's spot as Andrew Madden suggests.