Consider this; the 20K taxable payment for PAS is the delegated limit which Manning can offer without recourse to the tri-Service board. That's why it is so relatively low. Higher amounts would require buy in from all three Services and (guess what) it would fail because a lot of branches are suffering.
How, then, do you decide which is important? You must offer clear statistics and I'm not convinced the sorts of haemorrhaging statistics exist for the RAF pilot branch....yet. The issue is, once Manning is in full crisis mode for pilot retention, it'll be too late to solve the problem with a high-end FRI because it takes about 12 years to replace a pilot with 10 years experience (training added).
I've always thought FRIs somewhat devisive and imprecise. Instead, there should be an enhanced RRP scale to truly reflect the situation - like LOA, this can be scaled up or down with the times, accepting it won't drop below a set minimum to allow financial planning.