Benos can be plinked for small pure damage, because they have infinite, fast and perfect healing. There is also thing called "darkness" which means you can't shoot at shit you can't see and benos have ways to eliminate any flare and light bulb. No L42 marine has night vision and scope means they can't get rail flashlight, so they can't even see their entire screen without flares. Flares are also finite, especially on the frontlines, single marine really doesn't have space to take more than two packs (one in pouch and one in satchel).
Benos aren't slowed down by the damage, they are as fast at 1% HP as they are at 100%. Benos are also less numerous, which means its harder to hit anything, by just spamming. "Sniper" sentinel/sniper can just spam. While with sentinel its fairly meaningless, with actual damage, its not really nice. HvH during the night with scopes and suppresors is cancer.
This all happens if we go exactly as you say, because your suggestion is a straight upgrade to both benos, not a strain. Nobody cares about the armor if you can shoot offscreen from the darkness and unchanged onscreen spit just adds to being straight upgrade.
This could work however, all you need is to make Sentinel/Spitter "Sniper" glow like boiler, have need to "ground" itself to actually zoom out and then shoot (but still require actual fireline, not just sight). Then also somehow nerf onscreen shoots and in that case you have actuall strain that could be called "mini-boiler", or something like that. In theory it would be a nice way for baldos to learn boiler, while not taking boiler slot.