You merely assume that an aircraft could not un-perk once it was flown less. There's no reason why ENY values cannot rise and fall just like the American dollar. 
Then it will simply bloom up to where it was before, plus the newly perked ride's numbers. Why bother?
Example:
Spit XVI = 6%
Spit VIII = 3%
Perk Spit XVI:
Spit XVI = 1%
Spit VIII = 8%
Perk Spit VIII and unperk Spit XVI:
Spit XVI = 8.7%
Spit VIII = 0.3%
All you've done in yoyoed usage, and pushing one of the Spits higher than any fighter is currently.