Actually the ability you're thinking of is Rail Shot.
That change doesn't really make sense for the class fundamentally speaking since you're essentially looking to force a ranged class, that generally wants to keep as great a distance from it's enemies as possible, to have the main proc of one of it's specs tied to an ability that only has a 4m range. While it is an instant ability I don't think it would translate smoothly in actual gameplay.
Personally, it doesn't sting as much if/when Unload gets interrupted as a Pyro Merc, since the main reason I use Unload in the first place is to proc PPA, and whether or not it procs is determined as soon as the ability is activated. If I can get the full Unload off I just look at that as bonus damage, but that's just me.
Arsenal on the other hand suffers a lot more from interrupted Unloads...
I would only add in the ability for rail shot to proc off rocket punch along with unload and power shot. This would allow them to maintain their burst in melee where they are lacking due to interrupts. At range I agree there is no issue with pyro...but things break down when in melee and that is where the class is lacking. The abilitiy to deliver reliable damage when needed in melee range. Arsenal suffers because all of their burst is on the back end of their channeled abilities...if you can never channel you cannot deliver the burst.
On the first part of your reply I prefer merc over commando visually because of the multiple weapons systems the merc has vs. all commando shots coming from one cannon...commando just looks idiotic firing everything from one weapon. So I run gunnery/arsenal on my commando and and running pyro on my merc and he looks good doing it.