Wait, why do we need to buff pistols? Are we talking all pistols or just marine ones? My harlequins love their neuro disruptors. Inferno pistols are mildly suicidal, but they're only "bad" because haywire grenades are a thing.
Giving them bonuses to overwatch seems odd to me. I'm guessing you're going with the whole, "you can squeeze the trigger really fast" idea, but that doesn't really hold up next to, say, a shuriken catapult that fires a bajillion rounds a second. I'm not fond of the "fire your pistol and another weapon" suggestion either I'm afraid. Video games aren't exactly the best thing to look to for proper firearm discipline, and it doesn't latch on well to the existing rules.
I've never found pistols to be a problem. They don't shoot as well as a full gun because they're not supposed to. A bolt pistol is specifically designed to not be as good at shooting as a boltgun, but you gain an extra attack for holding it in close combat. Assault units that have them can fire off a round to soften an enemy unit up before assaulting. Tac marines can use their pistols instead of their boltguns when they want to assault that turn.
What problems are you having with pistols exactly?