I'm not sure the Vishnu is a great divorced wastegate design.
As you can see from the picture, is makes some pretty hrd turns, and then rams back into the main imediately before a reducer... Not good flow IMHO...
The BPM like I have is much cleaner, and merges much more smoothly. I assume the head end of the Vishnu has a splitter that fits into the turbo body like the BPM, but haven't seen the head end on it. Also, as far as I can tell, Vishnu doesn't sell the bellmouth any more, so perhaps they changed their minds on which was better since you were there Kevin.
Vishnu:
BPM:
More BPM Pics
I'm not sure I'd reccomend Vishnu stuff anyway at this point, just from a customer service perspective.
Bell mouths are good, but I would suggest one that extends the split beyond the first bend like the Cobb if yo uget one. Shorter than that, and I'm not conviced they are much better than stock.