Hi Peter,
I have no experiences to share with regards to the GSO RC's. Everyone is quick to give an opinion, I'll refrain from doing so as it provides little value.
Now to coma and field curvature confusion. I would recommend some research courtesy of telescopes-optics.net;
COMA
FIELD CURVATURE
You can obtain more information on optics from their site;
http://www.telescope-optics.net/
Cheers