I don't see a problem, you're reducing the damage you "would" take, a lv5 sentinel will be better than a lv1, and it also means now the weapon that's most effective against you is actually weaker than other weapons.

Have you seen assault rifles vs everything? Low TTK bruv, everything dies quickly, but that extra 10% on top is brutal. Am using the RR currently and when I come up against a heavy, it seems like forever melting the shields but once he's into armor he dies. the AR(Plasma Rifle) is the opposite, I'll rip into shields and then armor takes longer. These resistances would make the Sentinel better suited against weapons that are meant to drop it easily. instead of 110% damage, you'll take 95% damage, it's a fair buff.

It is actually 6.5% with the way resistances are calculated. Not much more, but a little more...

Not in my experience. I've been running RR a lot, shields seem to take forever and armor is just lol.
Anyways, let's say 3 damage mods = 30% for ease of typing this up, my Standard Breach AR does 51 damage, with damage mods it does 66.3, now against a shield it'll do 72.93 damage per shot. with the sentinel skill maxed it'll do 52.98, so to be fair you're actually shaving off nearly 10 damage per shot at this point.

It would be unfair to make it so a weapon designed to kill your race is weaker than any other weapon, this gives the heavy a buff but not one that'll make it better to use a RR over an PR against a shield tanked suit.