The field limits are set by either the field or the host...and they're what they are. That said...both of them can be influenced by constructive feedback by the players, and if they've got their heads screwed on straight they'll take good feedback to heart and make what changes they deem necessary.
In confined areas (i.e. indoor warehouses/close CQB/etc...) 300fps w/ 0.20s is completely sufficient...and you should really be converting to Joules to get a good sense of thing since we're talking about power and force vs. flight time and range of BBs. 350 is something put out there because it is (was) actually hard to find guns that shot under 300. Many shot somewhere between 310-330. The "cap" was put there as a convenience thing so you wouldn't "have" to downgrade your gun.
400 for CQB...sure. With known guys...and you'll get injuries. With new guys and random guys you will get more injuries and hear a lot more crap about it.
But it's ultimately up to the field/host. Personally I think that it's silly/stupid to have public skirmishes at 350+ in tight CQB.
And MED are bullshit. At BEST it'll be observed (and as noted already very often either the field or the situations don't lend themselves well to it). More likely than not MED will fail.
Since you're not going to be able to avert injuries by face shots, MED, etc...you're only controls are FPS, BB weight, protection (googles/masks). Another control often over looked at magazine limits and ammo loadout limits. The amount of "spraying" drops dramatically since there are simply not enough resources to sustain it.