Keeping single barreled guns relevant is just one of the reasons to nerf double barreled guns a bit. In case it wasn't clear, my proposal to limit the "double shot" to a standard action is separate from my proposal to slow down reloading but offer a full attack alternative which works more like Mythic Vital Strike or Pummeling Style (which are actually two pretty powerful options IMO)
The standard action for double shots is the sort of thing I could imagine Paizo actually coming out with in a FAQ even though it apparently wasn't the original RAI. Giving Gunslingers an effective full attack alternative in place of the ineffective current version of Dead Shot is pretty much just me daydreaming since that would be a pretty significant change.
Just don't forget that by making it a standard action, specifically, you'd lose VS compatibility. So you don't wanna be installing that nerf without being damn sure the rest of the revamping gets done (protip: they'll gladly do the nerf part, but the other bit would never get implemented) as otherwise it's just another nail in the coffin for firearm builds.