There's a big difference between being a 'gunner,' as you put it, and seeking a challenge. I can't imagine Dynamis is very challenging for you - or really, for any shell nowadays. These areas are what, 18 months old? Nearly 2 years? I'd be outright embarassed if a team of 40+ couldn't beat any of the cities handily AND have at least half an hour, 45 minutes to farm the zone. You say you take 50+. Sometimes a full 64. All I see from that is more people going home unhappy due to not getting an AF drop or their coin take being quite low.
Now consider the new zone drops. Relic +1 aside, those belts and capes are fucking FANTASTIC pieces. Nearly all of them are the best you can get for that slot, and they seem to drop from standard mobs.
There's really no excuse not to be past Diabolos anymore, and a dedicated effort would get your shell there (or close to it) in a weekend. If getting Relic pieces is the main goal, then neglecting some of the endgame best back and waist pieces for your shell in lieu of bumrushing the cities for armor that, for many jobs, is inferior to alternatives seems silly. Not only that, but you wouldn't have to turn as many people away - split into 2 groups of 32-36 and go in simulaneously. Maybe once a month, instead of the same old Jeuno or Sandy.