Both. Some have perks applied as a rule (262s, 163s, 152s) because of number of units (and the relative scarcity of the real aircraft). This simulates proportions in the arena comparable to real WW2.
Some have perks due to performance. The spit14 we have is perked because it has a rate of climb and a zoom capability that leave EVERY plane in the game gasping behind it. Spit16 might be more so. SpitXIV is perked for performance reasons (its an uber plane, basically).
Some have perks put on them due to complete de-stabilization of the MA. F4uC comes to mind. The entire arena was mostly c-hogs, and knowing the MA it was probably 95% HO dweebs and cherry pickers. Regardless, the plane was relatively rare (200 tops were made, and used only by a small group to shoot down kamikazes), so a perk was put in place after the fact, due to over use.
Basically, perks balance the gameplay. They can be put on for various reasons.
HTC has said they might revisit the 152 and unperk it, due to piss poor performance at almost every altitude that matters
(okay, so not their exact words, but you know -- I read between the lines!)
As such, there is nothing that needs balancing in a ta152. The 190D flies better than it so chances are that the arena won't be flooded with it (if unperked). It won't dominate unless super high alts, and then all the enemy has to do is dive to lower alts and it sucks again.
So the perks are a fluid thing. They change with the tide of the MA. This is also evidenced by the ENY value changes recently (spits getting ENY 10, some other minor changes).
