I don't see what would be the matter in carrying all three tiers. I mean, Demomans can. They can carry M79, M32 and pipebombs at the same time. And they're grenade launchers. Why would gunslinger be unable to do so, when his weapons are significantly both small and lighter?
The thing is related primarily to ammo. Even stock the handguns have a lot of ammo and will end up like the often spammed M14. (20 shots * 14 clips = 280 shots)
Looking at it, the 9mm has 225 rounds (15 rounds a clip * 15 clips) The hand cannon has 8 rounds a clip and 12 clips if memory serves me well for a total of 96 rounds, If just for test purposes we use the rev and give it 10 clips for a total of 60 rounds. We have a total of 381 rounds stock (without perk benefits)
Thats 21 more than the SS combo of M14 EBR and LAR. If the nerf of the M14 EBR reduces its ammo to 7 clips that gives the SS combo about 220 rounds, that still leaves the gunslinger perk with 161 rounds more than the SS and thats not including the following perk bonus mentioned in the original post
Can carry more ammo for handguns
Lets say we give the guns a 20% bonus to ammo. Unfortunately my math fails me at this point so I can't give an exact figure but its around 450 total ammo, now you see my point, ok its still less than the commando's AK/Scar combo which comes to around 550 rounds stock, but its miles to much for a crowd control/heavy target takedown perk.
A possible solution would be to only give the 20% ammo increase to the handcannon and rev's, which leaves the perk with a total of 412 ammo with all weapons (187 for just the hand cannon and rev), which still seems like a tiny bit to much a reason to carry all three tiers of guns.
TBH I suppose we can't do official tests of how that ammo will do unless the perk comes out. I will see how a pair of handcannons and 9mm's last ammo wise in a wave though and get back to you.
I think it'd be cool, but they'd have to implement 3 new weapons. What's the point of stealing weapons from sharpshooter?
Currently the SS has 5 weapons assigned to its perk making it have the most weapons for a perk. By taking away the two handguns it puts it on the same weaponry level as the rest of the perks, see one of my previous perks which explains this.