Whilst serving out my PVR-porridge at Binnsworth in 2003, I researched this for a chum who couldn't decide whether to stay or go and wanted some advice..
The basic crux is that, if you serve to '38/16', you fulfil your original terms of service. If you leave at that point, well, fine and good and here's the gratuity and pension.
If you ask to extend to 55, then presumably you would only do so if offered Spec Aircrew assimilation. Which includes a significant pay rise and a vastly better pension when you make 55.
If, having extended to 55, you subsequently decide FIIQ and PVR as a result, then your 'new' pension is as promulgated for a full-career officer who has elected to PVR. Your previous '38/16' terms being nihil ad rem. This means that in the first few years after your '38/16' point you may be significantly worse off if you decide to pull the B&Y. Don't forget, of course, that your pension is frozen if you PVR and is NOT index-linked until you reach the age of 55. It is then adjusted for inflation from the time you PVR'd until the time you reach 55. If that's 19 years at 3% pa, it will be a nice increase when you finally get there. But not until then...
Extending to 55 and PVR'ing a year or two later is probably the worst possible option financially UNLESS you've got a really good job offer waiting outside. Check the AP (it's available on the RAF intranet) and work out the point at which your 'post-38' pension equates to what you would have had at '38/16' - and don't forget inflation!
The only possible point you could use in your favour would be to prove that you were finacially mis-advised when you made your application to extend to 55. Otherwise, I'm afraid it's tough-titty, mate.
The chum I was advising had one advantage; the deskO had offered a 5 year extension (although not on Spec Aircrew terms), but the appropriate paperwork hadn't been completed - no signature from the applicant or recommendation from the Sqn Cdr as required by the AP and agreed by the Blunt Mates in the office. So my advice was that she (ahh..bugger, that's blown it) still had the option of the '38/16' exit as the 5 year extension had been purely speculative.
And, at the end of the day, the Blunt Mates at Binnsworth will do the best they can to help you - but you need to do proper research first, I would suggest.