Alright
I have a GM mas and I was wondering are there any performance benifits of having a "Pull through setup" (mas before turbo) and a blow through (mas before the throttle body) besides being able to vent to the atmosphere?

The best position is right before the throttle body. Several reasons for this;

1. Less restriction and less pressure drop before the turbo.
2. Quicker spool up of the turbo.
3. Any air leaks before the MAS in your turbo, intercooler and piping will not be seen (although they should be fixed regardless).
4. The MAS only sees the air flow that truely goes into the intake manifold and engine for more precise tuning.
5. You can vent to the atmosphere with no tuning or performance problems.

cool
now all I need is a hard upper intercooler pipe so I can get a blowthrough setup
thanks