I would suggest to make a custom filler plate if you plan to fill your Weesatch or Wasatch. I originally used a sheet of cardboard, but it felt too flimsy. I then used a sheet of matteboard, but that felt too unforgiving (especially when bending over, it would dig into by gut) I thus cut the matteboard in half and used it only on the upper section. This ensured less saggyness, as well as allow the flexibility of the rig without the matteboard. This was the most viable solution I could come up with in terms of comfort.
here's a shot of my Weesatch with the filler plate I created. It's freefloating, so you can see how it still maintains it's form instead of saggying like mentioned before.
There seems to be two types of people out there in terms of gear:
1. people that conform to their gear (get used to standards)
2. people that make their gear conform to them (makes his own standards)
I guess you should figure out which first before deciding on a rig. You may find that a non-modular rig is actually more suited for your needs if you're person 1, or you may prefer modularity if you're person 2.
As for Nortus's comments, I agree that having a full loadout can make you less combat effective in airsoft (due to others not fully loading out), but I also believe that you can adapt quite a bit with realistic loadouts. I'm in the middle in terms of the two theories of being realistic and being combat effective in airsoft, which I feel bad for at times, as I really would like to be totally realistic, but having to adapt to players that wear nothing but bdu's and a gun with hicaps, you tend to compromise. In the end, it's all for fun, hence why I've compromised.
Don't get me wrong, I have utmost respect for those that follow their impressions down to detail (EG: The Contras). It's all about the details with impressions, and when you start to compromise, you lose these details. It's just that I think I'd be running up walls trying to stay 100% dedicated to it, especially when I'm not on a team that supports it.