I g hopped my G&G T4-18 and it improved the spread a little bit but it was not all that impressive, you aren't suddenly going to be hitting an ear at 30 metres that I can tell. The same can be said for the PDI barrel added, it didn't make much of a difference at all. Both individually did help and the end result is more accurate than the gun came stock but I don't think it was worth the outlay, especially the barrel.
Then I changed the airseal of my S&T Tar 21 professional (cylinder head, piston head, piston, nozzle) and it made a sizeable difference to the guns basic accuracy. It definitely still has hop issues I can see that but airseal was a large part of the problem I had before and fixing it made a notable difference and I out shoot a lot of people now where I couldn't before. I am considering an R hop for it, mainly because it removes the need for the hop to apply the pressure and since the hop chamber is custom to the gun and plastic and not very good then an R hop seems like a good idea. Right now it can't spin a 0.28g BB and while 0.25g is what most people use a heavier one drifts less in the wind and is more accurate and consistent. So improving the hop should help on basic accuracy and past the flat trajectory part of the range.
My own perrsonal feelings so far on my limited testing is that my gun is basically accurate enough at usable maximum range, the projectiles run out of power and drop off and before that point I can hit half a torso with about 5 rounds. I would like that to be 1 round, but I doubt that is realistically possible. The question really is how consistent can the gun be near its maximum range and can a set of upgrades and tweaks really get the system to the point where you can get it down to 1-2 rounds at flat trajectory range and my assumption is that you can.
But an R hop can't possibly extend the range, all it can do is improve the accuracy up until the projectile hits the ground, or alterantively it overhops them to get that extra range but its much harder to hit with. R hops aren't about extra range really, given the same hop setting (flat trajectory then drop off) what they might give you is more consistency in the hop resulting in a tighter grouping at the maximum range. So if your current guns spread at maximum range is too large too be accurate then maybe an R hop and some other upgrades focussing on particular problems in the gun will bring that spread down to make maximum range usable. But its not physically going to extend the range, at 350 fps its always going to be about 30 metres of flat trajectory and a fall off of 1.5m (height of a person) at 50 metres. Whether you can use either of these numbers depends on consistency. Right now I feel 30-35m is usable on my gun, 50m isn't because they are all over the place and my hope is R hop might help with that.
Take a look at the ATP, as reference this is a link to the 350 fps page which is where these numbers come from and they match what I personally see out in the woods:
http://mackila.com/airsoft/ATP/07-b-07.htm