I've been able to use HPA with both of my Puma's without an issue. Make sure you are using a high pressure output reg though. Even a mid pressure reg (600psi) might not be enough.
And someone over on ICD-Owners.com posted up this link. If you're looking for a good paint to barrel match you could try getting a Freak kit. You might want to snap up one of these backs before they are sold out. Smart Parts Freak Back Aluminum Version - Bushmaster - Black