Hi there - I did my IR first (at BFC) and I don't think it really made much of a difference. The things you learn on your CPL will help you with the IR flying but the same is true of the reverse. By the time you get done with your IR you will be a bit more relaxed about the CPL knowing that the most difficult and expensive part is out of the way. Your flying will be sharper and your ability to fly accurate headings and maintain altitude will help you.
I also found that my confidence increased when the IR was over which definately helped my in the CPL. Also it can work out cheaper although I went over hours on my CPL to make sure I was ready and I didn't really save anything!
The learning curve wasn't too steep - it is all about the instrument flying and flying a twin - being super current VFR only single engine flying wouldn't have really helped me I reckon.