in my expirience, with right build, both sniper and assault rifles can tears through anything, pretty much. If you have something, that can combine both playstyles - that could be too broken.
for example, if i understand correctly, what you suggestin, is a sniper rifle with lower base damage and lower move penalties. But this could easily be just a plain better sniper rifle! because base damage is not so important when you get right perks
BUT - getting something like a bolt-action rifle, maybe as a legendary, sounds reaaly cool. Whith its own, unique playstyle