Hmm.. so ramzey's suggestion on the Spit5 is designating it to Spitfire F.Mk.Vb, instead of the F.MK.Vc we have..
I think it's a very good idea that deserves serious thought from HTC!!!

I think it might somewhat offset the overusage towards the Spit5, and perhaps pursuade people to use the Spit9 more... or even better, use less Spit5s at all.
Think about it... the summary will be something like this.
-AH2 Spitfire F.MK.Vb('42)-
Pros
* runs at +16 boost
* only 4mph slower than the AH2 Spit9 at deck
* maneuvers better than the Spit9
Cons
* much slower than Spit9 as alt is higher
* only 120 rounds of 20mm ammo(60rpg)
-AH2 Spitfire F.MK.IX(Hybrid)-
Pros
* much faster than the Spit5 as alt grows
* 240 rounds of 20mm ammo(120rpg)
Cons
* Only 4 mph faster than the SpitV at deck
* maneuvers worse than the SpitV
I think it's a nice equalizer between pros and cons between the two Spitfires, and it's not an external limitation such as a perk price, but a purely historic representation of the plane.
If you want more maneuverability at the cost of less ammo, choose the SpitV... if you want better high alt performance, and more ammo at the cost of some maneuverability, choose the Spit9..!
...
IMO, the current Spit9 should be removed the 'e' wing option, and should use only 4x 30cal + 2x 20mm option, since it is a 1942 Spit9.
Since HTC is gonna rework on 109s and Spits soon, this is a chance to request a new model Spitfire represantative of late '43~'44 time period which uses the 'e' wing options... and the current Spit9 should be designated as;
"Spitfire F.MK.IXc('42)."