The warmouth has a considerably larger mouth.
Have to disagree with you there. Warmouth and pure GSF have mouths that are very similar.
Ewest has posted, but I'm too lazy to search for and link, a Sunfish family tree, if you find it you will see that Warmouth and GSF are very closely related.
Find any photos of DIED's or my GSF and you'll find that their mouths are very similar in size and shape to WM.
Also not true that WM are much larger than GSF. From my research the record for WM is 2 pounds 7 ounces, the record that I found for GSF is 2 pounds 5.7 ounces and 12.5 inches from a pond in Kansas. I think it is safe to say that most folks catch GSF that are rather small but that doesn't mean that they can't achieve a good size.