Delta Wun Wun,
This is correct - if you already hold a JAR Instrument Rating (single or multi-engine), you are spared the instrument training in the JAR modular CPL. This reduces the course by 10 hours.
Most schools require you to do the CPL first, though, and IMHO there is a good reason for this: low-houred CPL holders are better flyers than low-hours PPL holders simply for having done the CPL course. This means they are less likely to need extra training for the IR. Doing the IR first could prove a false economy.
Of course if you are a high-houred PPL with plenty of multi-hours under your belt, the IR first may be a sensible option, however you will be in the minority!