Karanou
Just to give you my view on the subject. I am doing the IMC prior to any CPL or IR training, not because I consider it of much benefit to the CPL part, but I do think it will give me a bit of a start with the IR. The IR is the toughest part of the course from what I have been told, so any insight or prior knowledge I can gain beforehand will be to my benefit. The IMC will give you some experience of navigation using radio aids and you will do some holds and ILS/NDB?DME approaches which are all part of the IR course.
The CPL is still done mainly in VMC like the PPL, so the IMC won't be of much help except for knowing how to use the Nav aids. This should make the position fixing and the small amount of IMC flying that is in the course slightly easier.
Of course if I am way off the mark someone will put me right.