the only possible reason i can think of that LE would sound better is that you would get the version 7 mixer.
personally, i can't say it's worth the money investment for the computer (which you already own the dual G4)
with the mix+ rig you get TDM support, which is a big deal for your G4. i honestly think this is a well thought through path liam. if you see it increasing business, which brings more money in the door, i wouldn't hesitate to do it.
growing a business is always scary, but if you see a need, it's stupid to not fill it (in a well thought out way) and keep all these clients coming back and feeling taken care of.
from everything you've indicated, this is a move you need to make.
make it fit the budget, while maximizing it's usefullness. seems like you've done that.