Per some disability talks and articles (see solvecfs videos and How To Get On wordpress site, and @nyanko_the_sane
's posts on disability), CPET is accepted as objective medical evidence for SSDI. I am counting on it as part of my ongoing SSDI claim.
However, all your evidence will be reviewed in its entirety, so make sure you know what is written in your full medical records (ie, treatment or progress notes). As mentioned before, having a recognized ME/CFS doctor treating you and filling out appropriate SSDI forms is probably the most helpful.
If you are going to risk the PEM, make sure you get a 2 day CPET, not just one day. Otherwise, it will difficult to prove presence of PEM, and therefore, disability. If you are certain you do not have PEM, I wouldn't bother with 2 day CPET.