Next up, a very big centipede.
In a nutshell, way too weak for what it does. Dr 15/adamantine, mindless, can't fly, basically even worst off than the tarrasque. There is no way this is anywhere near its listed cr of 20.
The MM has the colossal centipede, which is cr9, albeit with weaker stats (24HD, lower speed, poorer str and only a single bite compared to the megapede's additional 5 tentacle rakes). However, I think it serves as a stark reminder that mindless vermin basically have no place in a dnd game past mid-levels, however impressive their physical stats are. Plus, if it is extra attacks you want, I can have a pseudonatural (ELH) colossal centipede at cr19, which blows the megapede out of the water.
Recommendation: I would suggest not using this at all, instead falling back on the default MM centipede. If you insist however, I feel another +2 cr would be in order (basically advancing the giant centipede by another 8 vermin HD), and assigning another +1cr for the extra attacks, defenses and str, for a final cr of 12. Though for the same cr12, I may as well just use the colossal scorpion.