"the most consistent thing about their mix is that they are inconsistent"

Too true, but there's not a lot anyone can do about that; the mix that's good for you depends on your specific keyboard, the voice you end up choosing for a particalar part and the situation in which you will be using the file.

Ultimately the only way to get a good result is to do it yourself. MidiPlayer does a very good job of getting the overall level of a midi file to a state where my PSR3000 is not exhibiting overload symptoms; after that it down to me.