As you also point out, you'd also have to do two lots of groundschool - firstly the IR exams and then the ATPL exams. This would mean extra cost and time.
The IR exams are not that easy.
You also need 150 hours pilot in command before you can be issued with an IR on the back of a PPL (only 100 hours with an IR on a CPL).
Do the ATPLs. Makes much more sense and costs less.
If you're keen on getting instrument time in your hour building, get an IMC rating and do some SE IFR hour building. I had 10 hours P1 IFR out of my 100 hours upon IR issue (which I passed first time).
cheers!
foggy.
[ 16 September 2001: Message edited by: foghorn ]