I have several OS Max 61's on various models. I find that the APC 12 X 6 delivers the best performance.

What you are experiencing isn't that wierd. There simply isn't that much difference between 3/4 throttle and full throttle on that engine. There should be some difference, mostly noticable in the vertiacal. But at cruise, you can't tell the difference.

Since you are higher up, you might try higher nitro which not only includes an oxidizer, but also the oxygen itself.