Why the red thumb? There is actually a lot of useful information in there. First question would be, what is your objective? I assume you are not trying to quickly downloop a kite while you are carving around on a sb? That would be easy, any wave specific kite usually turns on a dime ie fast and not with a lot of force. From what you describe you refer to tt riding freeride/freestyle looping. Which GTS do you have? I had a GTS2 and 3. Due to the pulleys imho there is a bit of a delay which is in the way of precise and fast loops. You need to make sure lines are under tension (btw you want this with any kite unless you like surprises). One way is to try the GTS on shorter lines. Worked well for me. And play with the settings. Also, the speed of the loop depends on when you loop i.e. where the kite sits in that moment (zenit, going up or down from a jump, power zone etc).
C and Cish kites are usually built in a way that the loop will be smooth i.e. consistent, with constant power through the loop. So that you actually get an acceleration. At the same time you want it controllable and predictable.
I am on Enduros now and the loops are ideal for my style/level. Found them more dynamic, consistent and predictable than the GTS. Clearly there are riders who make the GTS look like a looping machine. And it does loop nicely. What I am saying is it may not suit your style or the way you like to loop. Try fish poling the hell out of it when powered up with shorter lines, starting with zenit on the way down from a jump or during a backroll transition etc which should be a safe way of testing it.
Most bridled c kites should loop a similar user friendly but still fun way. Full on c kites will tend to have a wider arch hence have more power through the loop. And then there are kites which are meant to be advanced dynamic cross over machines e.g. Union which I found turns on the dime without any pull which is pretty useless. So I would not like it as I guess you'd have to slow the loop down enough to make it develop some power. Risk is it then slows down too much
As always a matter of preference. Try and play around with the GTS, if a younger version. The first one may be too delta-ish for loops?
Otherwise yes RPM, FX, Enduro etc. will be in that easy to loop but still fun shelf.