Lets assume, hypothetically I pilot a Goldfish with a Hwacha...
I take a buff hammer, and so do my three team mate, I buff the Hwacha, I fill up the meter on the left and the gun gains a 20% damage increase for the duration of the time, and then my three crew mates all buff the gun as well... The gun gains nothing more than the 20% I added.
Buff does not stack.
20% is a great increase, but not overpowered, it just separates a 20% difference of a ship with a buff engineer who actually buffs, and a ship with two normally equipped engineers...
Obviously there are many ways your crew can come equipped, but just because one crew uses the buff hammer, does not make it over powered.
However, I think you're not explaining your point clearly "mate"
You have shown a picture of the experience page and stats, which leads me to think you mean the experience which can be gained from buffing, this has been pointed out a few times by one of our community member Ceresbane (aka maximillian jazzhands)
The experience you can gain by being a buff engineer is quite high...
However since leveling up gives no actual benefits beyond a few dyes and cosmetics, I cant see how that is over powered, makes leveling easier but is not Over Powered.