Armour endorsements should be a percentage of the base HP

Before I begin I'll admit that I only have 10 hours in MNC and this is mostly theoretical with just a little experience.

According to the MNC wiki, Armour endorsements give you a flat increase of 100/200/300 HP for bronze/silver/gold respectively, regardless of the base HP of a class. This means that the assassin and sniper get double their HP by using gold armour, whereas the tank and gunner get a mere ~43% increase (not counting passive skill upgrades).

A common pattern in class based games is that classes with less HP have other tools (usually mobility) which allow them to take damage less often and for shorter times, and classes with more HP have less mobility and can't choose their fights as well, so they take damage more often. This results in every class having roughly the same survivability.

MNC seems to fit this mould to some extent. Going class by class:
Assassin (300HP): Fastest class in the game; dash and smoke bomb allow quick escapes; small hitbox means harder to hit.
Sniper (300HP): Doesn't fit that well. Generally fights at very long ranges where it's harder for the less accurate weapons to do much damage, and traps help to prevent people from closing this distance but after someone gets close the sniper has little mobility to escape.
Support (400-600HP depending on passive skill): Not sure here. The name seems to suggest he spends most his time around his firebase and teammates so he has other things to protect him, but I don't know much about the support playstyle.
Assault (500HP): Lowish HP but great mobility due to base speed, fly and charge (and the odd bombjump).
Gunner (700HP): Slow, even slower when firing minigun and cannot move at all when deployed. Large hitbox.
Tank (700-1000HP depending on passive): Slow, cannot move when deployed. Forced to enter close-ish range to do damage with jetgun. Charge helps to alleviate this though. Large hitbox.

The result is that the classes with lower health are able to get more out of the extra health from armour endorsements as they are better at avoiding damage, whereas the classes with more health don't get as much of a benefit. Modifying the armour endorsements to be a percentage of the class's base health means that the lower health classes get less health from them and higher health classes get more, which cancels out this effect. In practice it might be necessary to specially tailor the health bonuses for each individual class rather than using an across-the-board percentage increase.

The numbers you quoted seem OK in my book; an armoured Gunner has nearly twice the health of an armoured Assassin, and the Tank can survive a backstab or headshot.

Support is, indeed, a Support character. He should have a little backup at all times, whether it comes from his Firebase - Sentry Gun - or a friendly Pro.

Snipers on the PC normally run builds based around Clip Size and ROF, right? That makes it even easier to kill them if you can aim worth a damn with the Railgun or Mortar, or if you close the distance. Also, the Sniper is quite quick on his feet with a high jump. He's as fast as the Assault, IIRC, with Assassin having the highest base speed.

Finally, the lighter classes make a bigger choice when it comes to Armour. Tanks and Gunners have a reasonably high damage output without having to use ROF, Accuracy or Clip Size Endorsements, whereas classes like the Assault benefit hugely from ROF and the like. Therefore, the choice to wear Gold Armour has a large negative effect on your damage output, so it should have a large positiveeffect on your health.