I know this is not the answer you are looking for - but have you considered that if you do all the hour building before the ATPL exams, and then not fly for the six months or so that you are in school, you may lose any sharp edges that you honed on your flying skills.
Consider doing the laborious grind school first, then the hour building before starting the CPL flying course as a fresh and experienced pilot.
On a sad and scary note - if anything goes wrong with the ground studies - you wouldn't need those extra 80 hours!!