Hi Rob,
I don't think so as it isn't related to guiding at all or your guiding scope. The problem comes from your imaging scope.
A WO 0.8x reducer should fix it. Not 100% but stars will look much better if you use it.
Check this link
http://www.ewellobservatory.com/best...es/player.html
and select #15 "Field Curvature" from the "Outline" list on the left. There is a good explanation there why it happens.
cheers,
Mike