Vizard flow tested a bunch of mufflers back in the 70s - published the results a few places including Performance with Economy.
Glasspacks generally flow much better going with the louvers than into them.
Yes, the die that punches the holes "pushes" the holes outward... There is no burr from the punch side as there is from the side that bends/deforms when the punch goes through... Therefore less resistance to flow when you flow in the direction that the punch goes... No burr to overcome/go around...
When you make tooling for a part, you need to consider the draw of the punch...