Pistols really aren't that overpowered. They aren't very accurate, they take a while to reload, they give away your position, and the melee weapon that goes with it is absolutely terrible. Really the only nerf I'd want is to make assassins move much slower when reloading, therefore if you don't get the person on the first shot you're only option is run and hope to blend with the crowd to lose the guard.
On a related note, I really think weapons should just be made un-lootable, this would make it impossible to see the Axe+pistol+shield assassins that I have also seen (it also would cut down on accidental TKs, I've accidently killed multiple friendly assassins because they had a shield and I didn’t see their name in time).