Before we go about nerfing the buff hammer, shouldn't we first establish that in its current state it has issues. Sure you get the pro of extra damage. However you still have the cons of
Time spent buffing. During this time you are giving up other potential things you should be doing.
Short duration of buff. Requires timing and makes it difficult (though not impossible) to keep up during a fight
Use of a repair tool slot. A buff engineer has to either give up specializing his repair or rebuild or has to sacrifice a fire tool.
I think these make up for the 20% strength.
Time spent buffing, Sammy, you know most guns can be kept constantly buffed during reload cycles, and the fact that before most battles all the guns are pre-buffed to get a quick kill. Point is, it is still more worth while to run buff engi then to go gunner (sorry for bringing up class vs class Zill, but currently the buff hammer is a big selling point to not have gunners on a ship, so it is bound to come up.) Also, I would not call this idea a nerf, but a repurposing of the tool. Changing it from a tool to assist in kills to a tool to assist in defense.
Sorry, the buff engineer > gunner points being made here are completely non contextual. Yes, on the whirlwind for example, a buff engineer with greased rounds is a better option than a gunner in most scenarios. Well done. There are other guns and other ships and other builds.
Running a buff engineer does not replace the role of a gunner on a ship, a prime example of this would be a flak spire, 2 artemise and a mercury. If the engineers prioritize repairing over shooting the most likely result is a death that lasts slightly longer than if they did nothing, therefore it is perfectly viable to maximise the kill potential by running one engineer with buff kit and charged rounds to drop hull armour with the mercury, while allowing the flexibility of a gunner with 3 ammo types on the main gun to ensure a kill at any range and in any context. lesmok for the longer range, charged rounds for mid range arc finding and killing galleons (lochnagar won't one shot them, or wait, if the buff engineer drops the hull then jumps downstairs to finish a pre-buff it will.) lochnagar for everything else and extreme short range. The pilot should also be shooting, alternatively bring a flare gun.
Fix it when they're dead.
I can also list several examples of ship builds where gunners are essential and better desired than buff engineers with buff engineers being a 'nice to have but not essential' should you so wish: I thought it pertinent to post the spire example to point out that gunners and buff engineers serve different purposes and should not be considered different approaches to the same end.
Oh, I also agree with everything Sammy B.T. said.
The lumberjack speaks for itself.