EMac Truths go far, I bet McCabe can put them 375-400' without much trouble. I definitely disagree. If you throw mids 250' then you need to go up a disc class or two to get 300', as you said...going to something with more turn may net like 10-15' if the EMacs are too stable. But if you want to throw mids 300' it will take a bit of form work, but it's definitely doable. I think that mids generally want to be in that 280-300' range with decent form, it's a pretty typical range lots of players expect out of their mids. But that being said it usually takes some form checking, but it's really not that bad to get them 280'+ if you are willing to make a few adjustments.
Throwing a disc the same way for weeks will just ingrain muscle memory, you have to actively make adjustments and figure out what to change in order to get a distance change.
Sure, any pro player is going to throw their mid over 300 feet. I'm not sure how relevant that is to the average player though. Sure 280 is doable, and I've probably done it myself, but not with my (non-emac) truth, but with my evidence or something like that I get more distance because it turns a little more before fading. I haven't actually thrown an emac truth though, so maybe they are actually less stable and glidier than the normal truth's, in which case I'd agree with you.
In any case, I'd argue that the average player isn't throwing their drivers more than 350, in which case, 250 from a mid really isn't too bad. And I'm guessing you'll come back and say that if you only throw drivers 350 you should just used mids for everything, and then I'm just going to laugh.