If you're doing DO you can do grade replacement which I feel is a lot cheaper and faster at raising your GPA than a SMP/DO Masters. It's going to be a year or two either way (hopefully your score won't expire by then), and with a DIY you can still work on your EC's while doing well in your classes. Just retake all your C's or lower and you'll raise the GPA a lot faster than you could taking a postbacc.
If there's a particular DO school you really want to attend, maybe you could apply for their masters? I don't think a 2.7 is that low for a general master's degree at one of those school (I've had a friend with similar GPA get into the masters at a local DO school program) and they generally have a pretty solid chance at getting into the school of choice if you maintain a decent grade in it. But personally, I wouldn't attend a masters and just work on fixing GPA through grade replacement and applying broadly