I will say the Darwin's Danger Shield is probably the worst designed weapon in the game. To the point where I'd say every version of the weapon is bad. Sniper is supposed to be the frailest class in the game. Lowest healthpool, lowest speed of those low health classes which gets even slower when he has to scope. Unless you're a faggot and can quickscope any class from any range, you are the weakest class in a 1-on-1 close range fight.
This means that Sniper works best when behind his team, shooting from afar where most of the classes can't get to him and is able to shoot for longer and be the more effective overall. Regardless about how you feel about this gameplay, this is how Sniper was designed to function and has functioned for the game's entire existance. Sniper doesn't need more health because he has his team to protect him in most scenarios. Planting yourself by an Engineer nest really is one of the most effictive ways to play Sniper, if the Engineer can keep his buildings up, you will never die.
This is why I think that giving Sniper anymore health or resitances is a terrible idea. He's already usually safe behind a wall of players or buildings, he should be postitioning himself in places that put him at very little risk from any damage. Why bloat his healthpool even farther? It should be one of his major weaknesses, that when you can get to him, when you can attempt to chip him down, it's fairly easy. Sniper should have no options to increasing his health, period. In all honesty, it's the only weapon that I would elect to be totally removed from the game. Every version of it has been obnoxious. It is conceptually bad, Sniper, but with more bulk is always a bad idea.