Regarding PAS pay, I looked at it recently, and it seems to me that the PAS Flt Lt who accepts assimilation at the same time as his chum gets promoted will be behind in terms of pay all through his career.
Sqn Ldr + ER FP is about £1k more than top rate Flt Lt PAS. If the pair are both senior Flt Lts on ER FP, then on promotion the Sqn Ldr will be £4k ahead and retain that advantage until he hits level 9 and the PAS guy catches up, but never pulls ahead.
At the upper end, a PAS Sqn Ldr can out-earn a level 9 Sqn Ldr on ER FP by about £4k, but under what circumstances do we have a similar-vintage PAS Flt Lt out-earning his career stream chum by £7k?