Couple of points Kintyred, which make it even better than you described.
Even if you are not going to be a 40% taxpayer it works out more lucrative to leave early (on AFPS05). In fact even the gross figures are better. The second gratuity is also payable on death so if you don't make it to 65 you still get the money - well someone gets it. And the pension lump sum at 65 is index linked.
Do be careful though if you're interested in FTRS service as you would have to pay back some or all of the first gratuity and you would get no annual income (aka 'pension'). If you intend to FTRS leave at 55.