Agree with Tony, if you're happy with the Hyperions, and they're a great EP, you seem fairly well set with that range. Can't see you getting much use at 500x.
You could always look at some other accessory that you now can't live without
- maybe a UHC or OIII filter, perhaps an upgrade on your collimation gear, an *cough* Argo Navis *cough*..there's always going to be something else you'll think you will need