It's funny that the blue team's rides are what the red team's rides used to be in late war IL2 1946 matchups: Spit Ias or Hurris against 109s in CoD is like Fw190s against P51s in '46
I find it unreasonable to pull aircraft out of the set because another one got pulled (ie, straight up for balance reasons) but i also find it reasonable. No, i'm not crazy, it depends on the circumstances, let me explain.
The FMs are under revision. Since we can't have and use the accurate FMs yet, we need to keep the sim enjoyable for the most amount of people. That means we
temporarily choose balance/playability over accuracy, at least until the FMs get corrected. I guess this is why most servers don't allow Spit II's and E4s and i'm fine by it, because there are also other considerations in the mix.
I mean, think of the bomber guys too. How is any team going to win a map if the bombers don't have a chance? Currently they can't fly in groups or with escort, due to the memory leak bug giving people CTDs. So, how are the Blenheims supposed to reach the target alone against m-shell armed E4s, or how are the heinkels and stukas supposed to do the same alone against fast climbing Spit IIs?
I think what the server admins are doing is correct: since the FMs are skewed let's skew the planeset too, to have a scenario where at least the broader, relative balance of power is close to the historical one.
After all, that's what matters most in the mission oriented servers. Absolute performance stats are all that's needed in terms of 1vs1 engagements, but a server is more than that, it's an entire tactical engagement and the thought process is different: each team has to be able to attack and defend in a manner reasonably similar to how it happened back in the 40s.
When the FMs are corrected then let's go all out and allow everything.
I mean, if what we read in the history books is true, it would be both challenging and balanced in a way and it would end up depending mostly on player tactics: the spit Mk.Ia will be faster than it currently is, but if all the FMs are fixed the RAF will still have the better turning aircraft and the LW will still have the faster aircraft, because the in-game 109 is also going to get beefed up (it's slower than the real one).
Heck, the 110 is probably going to be the fastest one of all and get its missing m-shells too, not to mention that about half of the fleet during BoB had the uprated engines making it even faster.
As you can see, allowing everything with corrected FMs will be very similar to what we have now: the LW will still be faster and harder-hitting, the RAF will still turn better. I think that's what the server admins tried to do and they succeeded, to get the broader picture correct until the FMs are adjusted to historical values.