Yes and no. DOPMA does supersede, but the AFI governing Officer Promotions has a sneaky little caveat in the section titled Declining Promotion. If you are put on the promotion list, and subsequently decline the promotion (which sounds like a damn headache, by the way), then you will not be promoted. However, your name REMAINS on the promotion list. So practically speaking, you will not pin on the next rank, but you will also not be considered a "twice passed over" officer. It's a weird area, but in effect, you can't get out of your ADSC if you go this route.
Duck, I mentioned this before I think, and it's only anecdotal, but in our squadron on both of the last two Major boards, we had someone with a DP and no letter (common), DP and a DNP me letter, a P with no letter, and a P with a letter. On both boards, the proverbial "line in the sand" between who got promoted and who did not, was the DP v. P. In other words, the DPs with no letter and DP with a DNP me letter were all promoted, and the P with no letter and the P with a DNP me letter were not promoted.
I don't think this means that the letter carries no weight. I think it confirms what Learjetter was saying about the priority being on the AF's needs via the DP/P, which is seen as your CC's recommendation. It can only help though, in my opinion, if your CC's recommendation (P), and your own wishes (letter), are aligned.