Mand, My concern is with the "margin" of performance efficiency between the most preferred plane types and the rarely preferred types - which includes all attributes; speed, acceleration, ease of handling, armament.. etc etc
I was working on some basic comparisons analysis on the average speed of top 5 planes, the next 5, and then the next 5 planes etc. I worked with about 15 planes, comparing the current perk agenda and my sugested ones, until a freak accident blew my Comp main board and power systems, burning off the second hard rive too

(sounds like a lame excuse, but that really did happen!

)
Was too disgusted to put my hand on it again, but I do remeber some good results - the margin of speed itself, was not anything stellar, however, the plane types became quite different when suggested perks were applied.
For instance, in the current perk agenda, the top 10 planes in speed category(both MIL and WEP), are usually also quite easy in handling, great in acceleration, great in firepower and carries heavy ordnance. Usually those 10 planes have more than two, three of those attribites mentioned above.
All of their 'disadvantages' are usually covered up by advantages so powerful that practically the numerous mid-year war planes don't have much chance against them - of the many attributes, it is undeniable that the 'speed' category is often the most pronounced, powerful and evident one.
In an absolute comparison, it is no doubt the "new free Big Four(or big five, or ten or whatever)" with my perk agenda, are naturally easier planes to fight against, when seen from the cockpit of your average "1942~1943 plane".
Not only that, but their strengths and weaknesses are much more diversified(!).
For instance, as you pointed out, the fastest planes are almost all US planes - limited in WEP time, not so great in acceleration, hard handle in a pure dog fight. Of the four US planes in the top 10 speed category with my perk agenda, two are P-47s. Among the four, only one of the two P-47s really carry "heavy ordnance" - bombs and rockets, and those rockets are by pods not pylons, even. Meaning: the P-47 pilot will have to carefully decide on his configuration. Unlike the P-51Ds or P-47D-30s we have now, you can't just carry max ordnance, dump all and immediately expect it to become a fighter. Of course, the rest two, P-51B and F4U-1, are even more limited in jabo capabilities.
The next best planes are the three 190s - A-5, A-8 and F-8. The Antons are all limited in jabos. Performance wise the A-5 is balanced, A-8 is heavy in firepower but limited, F-8 is a dedicated jabo plane. These planes have twice as longer WEP time, accelerates better, and also has great firepower - though slower than those fastest American planes.
It is like this all along the edges. Most of the "best" of the free planes in my agenda, are strictly limited in strengths and weaknesses, and not a single of them have more than two, three strengths in the most important attributes of performance.
The roles are diversified, even their armament configuration and selections of ordnance, is diversified according to role(because of the limits in ordnance, unlike our '44 super planes we have). The situation where they can be proved effective, is also diversified.
The balance I look forward to, is a literary one, concerning many attributes, specific roles of planes, era of plane types etc etc.