Save it and wait for the new 300bhp CTR.
the more I think about it, this.
I love your CRZ. whilst its outward appearance isn't exactly to my taste, on the occasions I have driven it in anger, it has a great chassis and responsive steering which I think is on par with most current hot hatches.
I am a big fan of the seating position and the car certainly doesn't feel like a car for the market it was intended for. To me, it was the ideal car for you at the time, but realistically it isn't going to massively alter your opinion of the car at 140bhp or 200bhp.
summary: whilst the prospect of keeping a car you love and altering it to freshen it up is cool, I dont personally think that it will change it drastically enough to make you think that £5K was a wise investment.
Only thing is that a SC CRZ would be special. An SC CRZ with full mugen kit, even more so.
a new CTR is the logical step if you want to stay with a honda/looking for a hot hatch. but it wont have that specialness/rarity factor that the CRZ has.