Ross, depending on which type of tube you have, ACF of not, the issues at the edge of field might be the scope, eyepiece or both.
I've looked through an ACF or two and they're pretty good when pushing the max FOV...not quite as good as an Edge HD but decent.
If you're scope is not the ACF variety it could be the coma from the scope rearing its ugly head. Probably a better idea than spending a lot of $$ on a high-end eyepiece at the moment would be to take your scope along to one of the Sydney area events when they crop up, as there's always someone with quality glass around and if you ask them nicely they're usually open to scientific experiments
