I've always been told that if you decide to either do the IR or the CPL, you might as well bite the bullet and do all the ATPL theory exams.
The ATPL theory exams cover just about everything mankind knows about fixed wing flight. IR theory, CPL theory but also things like high-performance, high-altitude, turbine operations and a few other bits and pieces that you might need to operate aircraft such as a Piper Meridian or TBM850.
So with your ATPL theory in your pocket there's only a few scenarios in which you would need to do another theory exam ever again. (Switching to rotary would be the only one I could think of, or getting a typerating.) But with just IR theory in your pocket you'll probably find that in a few years time you're going to do something which requires you to hit the books again. And again for something else a few years after that.
The cost difference between an IR course and exams vs. an ATPL course and exams is negligible if you're thinking about buying a second-hand TBM850. Question is: do you want to invest the additional time?