Anecdotal, your mileage may vary, po-tay-to, po-tah-to, whatever, but here's my experience when I took the DAT a few years ago for what it's worth...
While I didn't bomb gen chem, orgo, or any of the other science courses I took, my grades weren't spectacular either. I had an undergrad total GPA of 3.0 and also a 3.0 science GPA.
It took me one summer to study for the DAT, with the 8-week Kaplan crash course, and on my own I did all the Barrons, Princeton Review and Kaplan test books I can get my hands on. In the following September when I took the DAT, I ended up with a 21 academic and 24 PAT. I've only taken the DAT once (the paper exam).
I think the key is to find a reasonable uninterrupted period of time to study, like over the summer, and take the test right afterwards while all that stuff is still fresh in your head.
The worst one can do IMHO is exhausting oneself with endless studying, then postponing the test for another few months to study some more because one doesn't "feel ready." This will only wear yourself out in a vicious cycle and even risk doing poorly due to exhaustion (you know this has happened whenever you start making a lot of stupid mistakes on the practice exams and miss obvious questions). NEVER, EVER LET THIS VICIOUS CYCLE HAPPEN TO YOU. You can only sharpen a knife so much before grinding it down to nothing.
Moral of the story? As long as you adopt a reasonable approach to studying for the DAT, it should not be hard to get above a 20.
HTH!