rich,
worst case, the IMC rating will make you a very bored student as you struggle to fill the sim hours for your IR with sensible things to do....
If you don't mind spending an additional 1000-1500 pounds during hourbuilding (for better equipped a/c, instructor, test, and fees) I would do it, but you won't get full payback in saved IR hours because of the minimum 55 hours (no credit).
Re hours - you need 200 TT and 100 PIC, so unless you already used 80 for your PPL and night qualification that is not a factor. Don't think the 15 hours PIC will set you apart from other "low hours" candidates after CPL/IR.
Ah, one more thing - I would recommend doing the IMC rating with proper screens or at very least a hood, not foggles. Screens give the most realistic "IMC" experience, foggles make things far too easy. Hood is halfway in between if you don't cheat.