I don't know about the MBox mods, but I have used their 192 I/Os before and they sound extremely better than the stock DigiDesign I/Os I own before I modded them..
And I tested their new 192s against the "Avid" ones,
and guess what:
Looks like Avid put the same I/O circuitry in the "new" 192 I/Os that they had in their 003s...
Black Lion's sound 10X better.
I would suggest getting a demo unit before you commit to anything, but I highly recommend them.
And, I know a mixer who was approached by Avid saying "We'll give you brand new 192 I/Os for free in your mixroom if you say you use them in a magazine article" and the answer was an
emphatic NO. And, he uses Black Lions exclusively on his line.
But, as always, try it for yourself and take no-one's word except your own to determine if you like it or not.