ENY are (more or less) and should be spaced on a logarithmic scale because what matter are ratios and not differences.

Wouldn't it be more logical/realistic to make ENY more of a fluid number? The ENY restrictions and perk bonus would work right into this way of doing things.

I was assuming the ATK rating would always have to be lower than the ENY rating, or it immediately broke in too obvious a way for me to see it as a real suggestion.

Yes, some of the high performers may need to be a little more regulated... but perhaps not as much as at first glance. Consider that the
Late War arena probably has some 30% of kills made by aircraft of 20ENY or higher. The takeaway is that many people
choose to fly other than late war aircraft in the LW arena, creating a little more dichotomy than might otherwise have been the case. On the other hand, early war fighters like the D.520

and Ki-43 saw action through to the end of the war...

IMHO, it would be unfortunate to have the P-51 carry a default perk cost. It was a very common front line fighter and obviously iconic. It should theoretically be available to newbies from sortie#1. That said, ~ 5% of
ALL MA kills coming at the hands of the P-51D (that's 5% of ALL kills, not just A-A kills) is a little ridiculous.
Been thinking: There is a problem with getting TOO lucrative regarding ENY if we have an enlarged scale as Bozon alluded to. Instead of taking 20ENY as the initial middle ground, let's make it 30 instead. The P-51D would then start at 14 ENY instead of 4. As it becomes overused, ENY drops, but never drops below 10- 10 being the minimum possible number used when determining perks gained for kills. In this way you avoid the possibility of a 2 ENY Pony providing an overly excessive 30 perks upon death to a 60 ENY P-39D, instead, with the shift, the max would be 70/10, or a mere 7 perks, keeping everything consistent with current accrual rates.
While displayed ENY never drops below 10, the logarithmic usage penalty formula continues to work in the background. When the 51D reaches a formula-derived 9.4 ENY, it will display as a 10ENY aircraft costing 1 perk. (Rounds up from .6 deficit)