My husband uses the Weesatch as well for airsoft. As Shinjit Noir says if you don't use fake plates or Rear Hydration pouch it does have a tendency to sag forward a bit.
As for fitting in the internal pockets. He can fit in 4 G36 mags. but then the outter 2 internall pouches are a bit useless. He just carries a spare battery or 2 on the one side and his car keys on the other.
THe fit is great he says and stays in place when tightened down for all the adjusting straps but he is 5'9" and about 190 pounds.
It does get a bit hot but any rig setup like this would get hotter under the collar.