This was somewhat addressed in a Soundstage review. My interpretation is that the difference between the new and old is subtle at best, and you would probably be best upgrading the powercord. The only downsize is that it a specialized powercord on the old Hydra, but there is a glut of these on the used market. Personally, I have not had a chance to compare. I am running an Elrod cord on my Hydra-8, but haven't done any comparisons.
http://www.soundstage.com/revequip/shunyata_hydra_8.htm
Rob
http://www.soundstage.com/revequip/shunyata_hydra_8.htm
Rob