Yeah, I'd leave it at +2 on a prof.
If you wanted to do something crazy you could go...
Manticore Tail
Proficiency +1
Damage: 2d4
Group: Flail
Properties: Versatile, Smashing 2
make it a real slaughterer's weapon, like a real manticore's tail. The 2d4 makes it like Brutal without adding the quality. You would be saying that it is very unwieldy, but hard-hitting. Maybe it's a bladed cube on a long, spiked cord....you might could even add reach...furthering the unwieldiness. Or perhaps it's two seperate pieces that clank together on a good hit, but I think that would affect damage more. Maybe you could even say that the Prof. is +2 if it's used two-handed (making it own the Versatile property a bit more)
Bah, but I'm tired and rambly