With MLS being so secretive about contract details, it's tough to know what the contract details are, but i'm pretty sure Pirlo is under contract for next season (or at least has an option, which he might not take) and Mix is not.
Mix does not hit the cap for $760,000 because he is not a DP and the max cap hit I believe is like $475,000. So we are using various mechanisms like TAM/GAM/DonBucks and such to pay his salary down. Still, combining everything he is very expensive.
However, if Pirlo and Lampard were not around, I would love to build a midfield to suit Mix. We have never seen Mix being Mix, because he has either been played out of position and/or in a shitty midfield setup. If given the right opportunity, I believe Mix would prove to be worth every penny.
you can't use TAM/GAM when discussing salaries counting against the cap. We could easily use that TAM/GAM for another player(s). He takes up $760,000 in cap space, one way or another.