Sceadeau pointed me towards a post on Reddit from someone who wrote down all their rep gain numbers from every master they found in a map. They tested all of the levels of maps and did the runs on multiple characters to see if that changed anything. Turns out no. Reputation can be limited if the quest is too low level for you but a quest doesn't seem to be able to be too high for you. Bringing people into the group doesn't change anything. The amount of reputation earned is strictly determined by the level of the zone where you turn in the quest to the master. (So finding Zana in a 66 map will only get you reputation for a level 66 quest even if she opened a much higher level map.)
There's also a table being populated on a wiki with reputation values by level of quest and level of master. It is unfortunately a very sad table. I was hoping that reputation would ramp up a lot when I got to level 7 with a master and that is sadly not the case. My current pace of reputation gain is what I can expect from the same amount of gameplay and it will take 4-6 months to earn enough reputation at that pace. That won't earn a tshirt at all!
The post on Reddit determined that the best way to gain reputation is to run low level maps without enhancing them at all. Just run them white and clear them as fast as you can. 50% of maps will have a master in them (as opposed to more like 1 in 12 regular zones) and once you find the master you can do the quest and leave. So you should get a master every 1.5 maps and if you can clear a map every 6 minutes or so (discount time spent on the quest) then you probably get 5 quests done per hour. You get 3678 rep from a level 66 quest so it would only be 560 hours to earn enough reputation to max them all (ignoring the issue of finding masters you've already maxed). That's only a couple months of playing 10 hours per day... That's at least viable...
Would you be better off running higher level maps? I actually have some clear speed data from last time around when I timed maps on my summoner to see where I was earning gem experience the fastest. I made a quick edit of that sheet to plug in the rep gain for the level of the map and no real pattern emerged. I suspect the level of the zone doesn't matter as long as you're able to do a good amount of damage. What will actually matter is the mods on the map. Temporal chains is going to slow down your 'find the master' speed, for example. But there is a bump in reputation from each level you go up so as long as you're still able to just run as fast as you can murdering everything it can only be good to go up in map levels.
So I guess the actual question is how much do you care about things other than just master reputation? If you're trying to maintain a high level map pool then you're going to be turning the maps yellow and that'll certainly slow down how fast you can clear them. But if you just blue them (or run them white) then there's no way you'll get enough maps to drop to keep running them. Maybe you could just spend a lot of money buying maps from other people and run them white?
I feel like master reputation is the primary thing that matters so aborting out of maps probably makes sense for me. But it feels so bad to be leaving potential map drops and experience behind. I don't think I can convince myself to do that on higher level maps. But I can probably be convinced that maps in the 66-69 range don't matter all that much so I should just plow through them for master rep.