They all use it & then say they don't - watch any PvP vid (before the mat storage increase); when they synth, they make a point of using 'basic' but always - surprise, surprise - have stocks of 'premium'...'funny that!
PvP is largely ego driven, remember & apart from being suspicious by the amount of damage inflicted, no-one can be really be sure, hence do you honestly think a min-maxing PvPer who is prepared to get the mats for thousands - in some cases - of G5 rolls for a few extra percentage points, will ignore the chance for 30% extra damage with zero in-game evidence of doing so?
I'll help you with the answer to that one...Hell no!
I used to use it - quite openly, but I'm an emergent PvPer (did I really just say that?) - on my multis after I found out that there was a bug where normal replen at a station would replace the basic ammo with premium if you had it synth'd previously. But now, meh...only the people I mentioned ^ can be bothered to do that because driving the SRV & idiotically, blasting rocks for hours is slightly less than thrilling.
Caveat: There may be one or two who don't out of principle & obviously in the PvP League, for example, there is the checking procedure...which shows they don't trust each other really, either[big grin]! But amongst friends & organised PvP, they probably adhere to the tacit agreement...but against players they dislike & hot-spot randoms, no - the name of the game is to try to get the bragging rights. Anyone who thinks differently is naïve.