The level of player required to use the P-40C as a an effective perk farmer will almost guarantee that it is the kind of player that already earns lots of perks. Most players will get more perks by flying MUCH more capable ENY 30 planes.
Now with that I can fully agree. I'd say the current ENY 20-30 range is the most 'effective' perk farmers available. And indeed you are spot on when saying that someone being able to score in a ENY 40 ride on a regular base will not be 'desperate for perks' in the first place
The main reason I swim in perks is that my regular rides are Mosquitos, P47D11/25 or F6Fs and I target mostly fighters, so ENY<10 are a large fraction of my kills. This nets somewhere between 2 and 3 perks per kill on average, before any multipliers.
And you are far above 'average' level

(which is constantly overestimated)
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I have analysed that with different methods before, and given the average ENY of killers and victims in the arena and the average k/h of the ordinary AH player, the net perk earn for him is in the range of like 2-5 fighter perks per hour of flight time. New players even less.
The new achievements gave me also an opportunity to take a peek at the peak amount fighter perks for random players. And as expected, few were "swimming in perks" at all. (Taking up the occasional Me 262 is a very expensive undertaking for most, considering the jet's average K/D of 5-7...)
With a more extreme ENY scaling, we would probably end up with more perks for the 'better' pilots, which would benefit most from it. The question would be if the anticipated increase in usage of 'lesser' rides would offset that change in overall perk balance.
And that's not entirely hypothetical - We had been there before, until autumn 2006 we had exactly this bigger ENY range, with for example the C.205 being at 40 and Hurri I at 60(?). (I made my top record perk farming sorties back at that time

)
But even back then, the usual supects (like P-51D, Spit 16, La-7, N1k) were making up most of the arena.