Epiphone Les Paul vs Gibson Les Paul

That's like asking how long a piece of string is. It can vary hugely from instrument to instrument. I have played some "low end" Gibsons that punched above their weight, some which were pretty much on a par with a good Epi. The same with Epis, some that were planks and some which were really quite nice. The big difference will be the sustain, a proper LP will sustain for a good 20-25 seconds when you pluck an open low e. Epiphones generally don't do that. This is mostly down to the quality of the wood.

Don't sweat it, your guitar sounds like it is more than up to the job of playing live. The pickups are quite a big part of an electric guitars sound anyway, so it will sound good (possibly not great though owing to the limitations of the instrument) with the Seymour Duncans. The tuning should also be stable (hard to sound good it you can't stay in tune), so you should be ready to rock.

Also regarding those Gotoh vintage locking tuners, have a look around on line and see what guys have to say on thegearpage etc. I was shopping for locking tuners for a les paul recently and was considering those as well. They are not universally acclaimed. You might be better off going for Schaller or Sperzel. The Schallers are 16:1 while the Sperzels are 12:1. Both work fine, but the Schallers are a bit tighter and more accurate.

That guitar should be fine for gigging after the mods are done. The 2 biggest complaints I have with my Epi LP is the bad pickups and it doesn't hold it's tuning as well as I'd like. With your mods they should be rectified.
I wouldn't be too concerned with comparing it to a Gibson or not, Gibsons vary wildly, some I've played were just awful and some were grand. As long as your axe hold up well and sounds good then you've nothing to worry about.