If I were you, I'd get the testing done if for no other reason than your own sanity. It was frustrating for me as a newbie when no matter what I did, Menace's BG didn't move. I can only imagine how much more frustrating it has to be for you when you are a seasoned caregiver with a cat that has already been in remission and you just keep hitting a brick wall. It's frightening to keep increasing doses and see no results and I found just knowing what I was up against allowed me to feel much more comfortable increasing until we got to the break through dose.
Chances are good your vet has never knowingly dealt with either condition (not all acro cats need large doses of insulin) so if you can convince her to do the testing and it confirms either or both are at play, she'll probably be totally out of her element. I wouldn't hesitate to pointedly ask about her experience with either condition to essentially back her into a corner as far as getting the testing done and dealing with it if either/both are positive. You've increased and backed up and haven't been able to find that sweet spot so I don't know that there is much she can suggest in the way of dosing that is going to differ from what you've already done.
I know medical personnel can be difficult to deal with sometimes because they don't want to admit their own shortfalls but I also think that a well positioned/presented argument should lead to a partnership as opposed to a dictatorship. If it doesn't, then I'd be shopping for a new vet because she should have more respect for you and how you have managed C.C. up till now.