First, a bit of analysis on the traits as they exist now:
MRM: This requires Finesse, which is the best non-specialized damage-increasing trait in the game. Compare reloader to finesse. Reloader offers a 20% decrease in reload time compared to finesse at 15%. No one complains about the lack of offensive power in shotgun masteries. Finesse gives you 75% of the bonus but is usable across all weapons, and after you have invested two points, the third point becomes pure awesome with a 45% decrease in firing speed for all weapons. Second, having points invested in finesse means you can usually get whizkid right around the time you do hell's armory, which will give you easy access to assemblies that WILL increase your damage.
As far as MRM itself, I have to test this out, but the thing is it will allow you to run in circles around slower enemies. Hellrunner and dodgemaster help you run *away* from an enemy, but a) the accuracy penalty makes non-shotguns too slow to fire and b) weapons generally fire slowly enough that, even if you hit the enemy, you will generally get hit in retaliation. But if you are running with MRM and fire a fast weapon, it seems likely that you can fire and start moving again before your enemy retaliates, and once you start moving again you are still running. Theoretically this should allow much better active defense than either finesse or hellrunner.
Also, while it blocks SoB, it does NOT block EE. EE is a significant damage increase for chainfire weapons - I dare say much more important for enemies on the edge of vision. There is no reason you have to beeline for your master trait - taking 1 or 2 levels of another helpful trait can really increase your gameplan. EE also allows you to focus on mods/assemblies that add damage instead of accuracy. MAc suffers in the damage department unless you get a lucky S mod or laser rifle. EE builds can go for e.g. burst cannon assembly.
I personally would like it if the master trait itself got a SLIGHT buff, but I don't have any ideas that would add a unique mechanic and not simply copy another existing trait.
Msv - briefly, it does not block SoG and EE. 5 levels of SoG turns any character with a decent pistol into a juggernaut of destruction. The only limit is reload time, and 10mm chains can help. You can go for this trait and put everything else into SoG. I think it would be fair if you took 0 damage 60-70% of the time. In terms of late game enemies, this master trait really only effects arachnotrons, and even with this trait and 70% chance not to take damage, a group of arachs can chip away at you pretty quickly. And as discussed with runningman, EE can boost damage significantly.
My problem with this trait is that it adds little survival against the toughest enemies. Compare it to e.g. berserker, entrenchment, MMB. I would rather have a passive 10-20% damage decrease against all damage than anything else. Another interesting option would be to reduce the damage taken by armor. Or to have large health packs heal all the way up to 150% health even if the character is at 1% health to begin with. Of course, I think this passive damage decrease in addition to the 70% chance not to take damage would be too powerful. As it stands, the weak enemies that this trait will work against are almost no threat in the late game anyway, and it offers no help against the toughest enemies that laugh at your 5 points in tough as nails. What I want to see with this mastery is something to increase defense / efficiency against the nastiest enemies.
I think it would see more play if it added a bit of extra damage, but I think that would detract from the unique character of the trait.
MEn - interesting trait with a balance of defense and offense. I think it takes a bit more planning to use it properly than most other masteries. If your armor absorbs 0% damage of the relevant damage type, then you are not using it properly.
The thing is to abuse the % damage reduction by stacking it with other high % damage reduction. Fireproof armor + MEn = 75% reduced fire damage. Cerebrus armor + MEn = 100% reduced fire damage. Ballistic armor = 60% reduced melee damage.
Also, the trait does NOT block eagle eye. You can get a lot more efficiency out of your chain fire weapons with a point here. With regard to skarczew's comment - a MEn character with a point or two in eagle eye is going to do more damage than an ammochain character.
One minor buff would be to add a little more % damage increase or damage reduction after say 2 rounds of chain firing. So by your third round you are really dug in and get e.g. an extra 5 or 10% damage reduction or an extra 1 or 2 flat damage reduction.