I think when it comes to next gen IR missiles, we have to assume the seekers on them are going to be worse than their real-life performance, otherwise gameplay goes entirely out the window.
So assuming that, the jump from the 9M to missiles like the AIM-9X, ASRAAM, IRIS T, and Python 4/5 is going to be comparable to the jump from R-60M to R-73 at least, however when we actually look at what that level of increase gets us, it can be as low as a 0.3 BR change, looking at the MiG-29 9.12 - MiG-29G
So given that IR missiles are generally a secondary option for fighter aircraft, and that even with next gen IR missiles, radar missiles are still going to be the primary option for most people, I wouldn’t be surprised if those missiles alone only added 0.3/0.7 to the BR of their airframes, you then add better radar missiles onto that and you’re still only looking at a 0.7/1.0 BR increase.
Consider the difference between the F-16ADF and the F-16C, you have a decently improved IR missile and a vastly improved radar missile, now if we apply a similar change, an F-16V with a decently improved radar missile and a vastly improved IR missile would again see a 1.0 BR increase.
Now obviously there are other differences, radar can be included with the radar missile, as at least in air RB that is its primary function, there is the addition of HMD as well which factors in as well as an increased CM count, but if we then include those, the BR difference directly correlated with the better weapons is actually diminished.
There are other examples that can be pointed to, again the MiG-29 9.12 compared to the MiG-29G probably showcases it best, that a vast increase in the capability of at least 4/6 weapons only warranted a 0.3BR increase. The F-15A compared to the F-15C also highlights that a vast increase in lethality only corresponds to a 0.7BR increase.
By this logic we should actually start seeing either new IR or new radar missiles by BR 14.3 and likely both by BR 14.7