I think you'll be fine with your wing joint, as long as you made sure the plywood joiners fit well and use plenty of 30min epoxy for the joint.

I did fiberglass the wing center joint. Whether this is necessary seems to be a matter of religion for some people, so I will just say that I personally prefer to do it. It can add a lot of strength to the wing with only a negligible amount of weight gain. Plus it really isn't too much work. Mine will be moderately overpowered with a Magnum 1.20FS, so it's better to be on the safe side.

The GP instructions does state that if you plan to put the plane through some serious aerobatics, you might want to reinforce the important areas. They, of course, expect you to know what and how to reinforce. Good thing is, you can always go back and do it later if you so choose.