That is a fair request, but not easily answered. Best you can say is "you would know it if you saw it."
So you can only give me an example of an unbalancing airplane, and not actually tell me what "unbalancing" is itself. Are we in a Socratic dialogue?
In the past, I have suggested that ENY/Perks be based on K/D ratios and, or, use, and the suggestion was roundly rejected. Ultimately, the rejection is correct: K/D ratios are easily skewed by who uses an airplane, e.g. P-38J, and popularity is skewed by factors other than performance, e.g. P-51D.
We are still without a working definition of "unbalancing," but the term is used all of the time to say this should be perked, and that not. It is all ad-hoc.
My only reason for saying the XVI should be perked is that the F4U-1C is perked. They are different aircraft, no doubt, but their strengths balance out, even to the slight advantage of the XVI. But I wouldn't care if we unperked the F4U-1C. I don't think it would cause armageddon in the main arena, and at least it would be a step toward consistency and I could stop saying the XVI should be perked.
You want the Mk XVI perked, you need to justify it. Nobody has ever made a valid argument for it yet. Saying it is "double plus good" is just BS hyperbole.
I've given a valid argument that you simply ignored. But here it is, super simple:
The F4U-1C is perked.
The XVI is as good as the F4U-1C.
Aircraft of similar performance should both be perked, or not perked (fairness principle)
Therefore the XVI should be perked, or the F4U-1C should be unperked.
QED.