You guys may think I'm joking, but I'm not. When I took that first diag back in my PR days, I got a 4 (1V-2P-1B)!!! Granted I didn't know that I shouldn't have left any answers blank, but a 4 is still pretty bad. But, after that course, I went on to get a 27 on the real one. Then, I signed up for the PR course again but studied mostly on my own and got a 30 on the second real test.
What I'm trying to say is that even a ******* who scores a 4 on the first diag has gone on to get a respectable score (27) just ten weeks later. I will say that I worked my butt off to do that. I worked on my reading skills and read the Science Review inside out. So, please do not panic after a low first diag score. Just work hard, do the Science Workbook diligently, read the Science Review thoroughly (except maybe the last chapter in OChem), and you will be fine.
Also, I never scored higher than a 24 on any of the PR diags. Somehow, I was confident that I wouldn't be screwed for the real test and that things would work out, because I just knew that I would have a completely different psyche going into the real test. I didn't know that prior to the first real MCAT, but I was sure about it before my second MCAT (on which I got 30). For the PR diags, I didn't wake up 2 hours before the test like I was supposed to, wasn't able to have the same sense of urgency, and the laidback, informal testing atmosphere wasn't conducive to focusing. I'm not saying that you should be complacent with low diag scores. But, I think there is significant difference between taking diags and taking the real test, so your diag performaces may not be the pinpoint correlations to the real test.