My feeling on the issue is that if people would wear the face/ear protection that comes with their goggles, mercy distance would be a non-issue at any FPS. The face/ears/mouth are the only areas that could be damaged to any real extent by a plastic airsoft BB of any weight. The rest of your body can take a few hits (This does not condone "over" shooting). Sure it is going to sting/hurt a bit, but I bet you will watch your back and flank areas from then on. If you cannot stand the "limited" temporary pain of multiple hits from a BB going approx. 400+/- FPS, I am not sure why you are trying to recreate the military "feeling". Without the pain factor that causes adrenaline it would be boring and without incentive not to get hit (just like real life). I realize everyone wants to wear just the goggles to look as "Milsim" as possible but to counter this you can camo your goggles to match your uniform with a little ingenuity (Problem solved). I never play without full-face protection, I expect to get shot in the face and plan for that.
Cry Havoc, Let slip the Dogs of War!
"Opportunities multiply as they are seized."
- Sun Tzu, The Art of War