If it isnt an OHK, then the headshot damage increase would be really whimpy. 120+20(BH)+20(Medic Buff) = 160
If a headshot with the Barnett cant do 160, then that means its less than a 1/3 increase in damage. Wouldn't even be worth it to aim for the head.
This is what I'm figuring, meaning that a light taking a HS from the Barnett is likely going to kill them no matter what, and a light taking a body shot when he's unbuffed will do the same. Looking at the difference in damage from the Barnett to the Drog I would say it's fairly clear they know that the Barnett isn't going to be a weapon of choice for the majority of players and so they're okay with the damage boost.
I would love more concrete numbers on all this though, since what you have estimated would mean that a HS would drop an unbuffed Heavy as well.
How do we know these aren't headshot stats?
Devs specifically stated you'd need to headshot an unbuffed Light to kill them.
Base damage is more useful information than HS damage, so they usually make base damage their primary focus when writing guides. Normally we get a "HS bonus" or "HS Multiplier" stat.
And the information about the HS to kill an unbuffed light may be outdated, or was one dev saying one thing when another had something else going on. A lot of people think they said "no snipers" but clearly there are snipers, and others heard "No OHKs" but they've found their way in as well. Information gets interpreted in different ways.