From what I've read and heard it appears as if there is no downside to signing up to 60. Some T&Cs and the start of AFPS15 will happen whatever you do. If you don't sign up you are taking the risk that they still want you at 55 - for no gain that I can see.
Viz - put your situation through the pension calculator. I'm AFPS75 and I get my lump sum (and a pension) whenever I leave. The AFPS15 portion gives you some when you leave as well as some pension, but not the full deal until State pension age (68 for most of those here - subject to change!!).
DITYIWAHP - I think that sounds about right.
High Spirits - run it through the pension calculator choosing the "not PVR" option. I don't think that you are any worse off (or better off) if you've signed on to 60.
Bottom line - signing up to 60 has no effect on your pension should you leave at any time before 55. It has a positive effect after that because you are working longer and a big benefit if you stay until 60 as that is the age AFPS15 is designed for - THIS IS WHAT I THINK, I COULD BE WRONG (it has happened before, don't tell the wife).