I'm also 4x vega 64's, 1990 or so (though i am on lc if that matters).  You can definitely get that wattage down if you care.  I had zero problems with the reg mod (from hella).  Definitely worth a shot if you have an hour or two to spare. 
I've yet to test the new stak, but want to get that cpu up and running as well soon.  Hopefully push out another hundred h/s and check if it's worth it.