I have experience flying the exact -12 tail mod you are contemplating, and while not terribly short on hours in it (probably a couple hundred), it is minuscule compared to the man who offered you the advice you rejected.... I think his suggestion to leave well enough alone was the 'cliff notes' version relating to this particular mod. His experience flying modified airplanes spans a lifetime ;-)
My personal opinion, may vary from others because I would not be looking at the equation from a 'working bird' view. The mod may not be enough of an improvement in some folks minds to be worthy, but if you're starting from a rebuild and have easy access to the tail feathers and a path of installation, it will certainly be noticeable in the balance and control pressure departments.
Side note for a parallel comparison; I have a pair of Thrush aircraft, same size, same engine. One has a fresh engine, fresh over all rebuild down to the paint. The other is pretty much timed out in every aspect, but it has a modded tail that doesn't change the hauling capabilities, it simply eases control pressures. A lot of other facets are affected as a result of that, but those are beyond the scope of your concerns here... Being a single pilot operation, I could pick either on any given night. Probably don't need to tell you which one I climb in almost every time ;-)
I like a good flying airplane. And I like a better flying airplane more!
This mod isn't new, finding 337's will not be hard. It has also been done to J3's, but there are plenty of 12's with that tail, both restricted and Std. You probably could never make developing an STC for this pencil out, as most people that are spending serious money on a 12 for mods will end up with the -18 tail.
Good luck!
Take care, Rob