No, it isn't, but you keep arguing from a standpoint that the P-51 is under represented compared to the Spitfire and Bf109 (your choices of contrast) and thus people who know better and are argumentative picked up their keyboards and responded.
I have no issue with the idea of more P-51s, but I do have an issue with people making claims that are false.
And no, there is no "equal representation"; true, the Merlin Spits also lack 150 octane fuel, but the Spitfire Mk. XIV is modelled, and the BF 109K-4 is modelled. Both are late-1944 birds that the P-51's in the game simply can't compete with, but could historically.
Yes, we have the Mk XIV. Spitfires were more varied than P-51s, deal with it. As a percentage representation the late war Spits and P-51s are about equal and if they added an Allison engined P-51A the early war would also be matched up. Actually, thinking it over, I think the P-51 has a higher percentage representation than late war Spitfires.
FYI, the Spitfire Mk XIV is also running on 100 octane, not the 150 octane that it was quickly rated for.
And no, the P-51 could not compete as a fighter with a Spitfire Mk XIV just as the Spitfire Mk XIV could not compete with the P-51 as an escort. A Griffon 65 running on 150 octane puts out more power than any WWII Merlin and the Spitfire XIV is significantly lighter than the P-51D. You do the math.
An example, the RAF used Meteor Mk Is, Tempest Mk Vs, Mosquito Mk VIs, Mustang Mk IIIs (D Ponies), Spitfire LF.Mk IXs and Spitfire F.Mk XIVs to intercept V-1s. Only three of those could do so without diving to get overtake speed, the Meteor Mk I, Tempest Mk V and Spitfire F.Mk XIV. All of the piston fighters on that list were running on 150 octane for that role. This information comes from an RAF Mustang pilot who said he didn't know how fast the +21lbs boost Mk XIV was, just that it was faster than his Mustang.