Frank-a upgrades to Frank-r, as previously mentioned.
A relevant aspect of the upgrade is just the fact that most of the players use the "2nd best MVR band" HR.
Under the provisions of this HR, the Frank-a and the Frank-b are limited to 31k ft, while the Frank-r can go little above 38k ft.
Now, it is still to low to counter P47 (max alt 42k ft), but it's extremely useful against basically any other plane.
If people are still using this HR they should stop immediately. It's really pointless and skews combat results (as you point out) to advantage only a few specific airframes at different times of the war. Useless. Just use layered CAP and tactical adjustments based on your opponent's style.
For example, you can sweep very well with the Frank-r, putting pressure on the allied player to either use his P47 in sweep or to cover his bases, since the various P40s will die in droves to Frank-r sweeps.
For the very same reason, it's an excellent plane in defending against whatever plane is not a P47 due to the fact that they arrive at 31k ft and you dive on them.
I am a great proponent of low alt CAPs to counter P47 and still I go for the Frank-r due to its power in sweeps.
It's anedoctical, but I confess that, on average, I found the results of the Frank-r much much much better than the ones of the Frank-a. Don't ask me why, since the stats are extremely similar, but that's it.
As Alfred points out small differences are big in this game. So many players look at speed but forget manoeuvre and each band.
Frank-b is a whole different plane and I use it mainly as anti-bomber when I do research it. Its problem is again the fact that it can go to maximum 31k ft; the 4x20mm cannons look great on the paper but it's not that good as a sweeper as the Frank-r or Tony-II, which benefit from the dive.
In one of my two PBEMs I arranged my R&D to have the mediocre Tony-II (which, just as the Frank-r, can go above 38k ft) and the Frank-b. One is the multirole SR=1 fighter used everywhere, the other is the anti-bomber guy.
Again, forget about the altitude restriction HR and your discussion is entirely different. The Tony-II a sweeper?
The Tony of both variants is mediocre in any role, and late the only truly useful sweepers are (in order of preference) the Ki-83, the N1K5 and he Frank "r."
I perfectly agree on your opinion on the 2nd best MVR band HR, however I have had to play under it on almost every game I played.
Under that HR, the Tony-II is a very good sweeper due to the dive advantage and the easy research line, which helps a lot in having it early and making mess in '43.
In one of my two current PBEMs I decided to go for every plane I have always forgotten and I am basing my airforce on the Jack and the Tony-II. It's probable that I won't see neither the former nor the latter due to A.V. on Jan-43, but I am appreciating a lot the easy research line of the Tony compared to the one of the Frank.
In the other PBEM, which is also going to be a A.V. but on Jan 44, I went for the Frank-r as usual and I found myself in some small troubles due to a remarkably slow R&D. So, in general, under the HR about MVR bands, I'd say that the Tony-II is somehow equivalent to the Frank-r. The Tony-II makes up for a much worse plane in late game but it arrives early and helps a lot in 1943. The Frank-r is a very good plane for late game but, if it arrives late due to slow R&D, it leaves you a little bit under pressure in 1943.
In general, my way is to go 'all-in' on the final George, the Frank-r and the Oscar-IV. I rarely use other combinations. Whatever the HR regarding altitude are.
I found, from my very little experience of it, very good the Frank-b, albeit limited by a very unfavourable research line. All considered, I'd never trade the Frank-r for the Frank-b, though.
As mentioned, if we forget about altitude restrictions, I'd still go for the above combination of George, Frank-r and Oscar-IV. But that's just my humble opinion, possibly obscured by gross incompetency and my blind love for the Oscar line.
I do also recommend some production efforts on the Tojo, especially the -IIa. It arrives early and makes up for a very decent SR=1 fighter until mid-43.
To remain in-topic , my opinion is fairly simple: yeah, the Frank-a doesn't upgrade to the Frank-b and the Frank-r is a better plane than the other two of the family. So, I almost always go for the Frank-r as my SR=3 fighter for late game.