Ginseng,
Having a Flt Lt receive a larger pension than a Sqn Ldr clearly is not untenable as far as MoD is concerned, since under AFPS05 a PAS Flt Lt can achieve a larger pension than a CS Sqn Ldr. As you say, who wants to get promoted now?
That's exactly my point. I would suggest that the answer is; only those people who are sure that they will get promoted beyond Sqn Ldr or who have no intention to staying until they can get a full pension.
If you are still a Flt Lt aged over about 33 and like the idea of staying in the RAF until you're 55. My advice would be to reject promotion and hope for assimilation. Mind you, watch out because not many Flt Lts are being assimilated these days. I suspect that, from now on, entry to the PAS will primarily be used as an incentive for CS sqn ldrs to stay and carry out alternate flying and groundtours, or to retain aircrew with important additional skills such as QFIs and QWIs.