Actually... After re-reading the VI Advance script (the closest in accuracy to the original), I do feel its possible Kefka was actually using Gesthal to gain power and did plan to betray him. Kefka is the first to mention the Warring Triad and he mentions twice (One in the Magitech Research Facility and the other time in Thamasa) that he was purely collecting power for himself and he planned to "revive" the Warring Triad as well as build his own empire.
The dialogue for the scene on the Floating Continent is different from the SNES version. One, it points out that Gesthal had no plans of "reviving" the Warring Triad which Kefka was trying to do, but a few lines change. In the SNES, Kefka says this after Gesthal fails to beat him.
Kefka: And now, Statues! You've shown me a sign!
It is time you show this old man your true
power!
In the more accurate script he simply says...
Kefka: Triad! Your first victim has stepped forward!
Show this useless old man your true power!
The first indicates Kefka doesn't know what he's doing and believes the Warring Triad is showing him a sign cause he figures it out, the second gives no inclination in fact Kefka's lines before it shows he may have known about the Warring Triad's neutralization field which is shown by another line change...
When Kefka goes to use the Warring Triad to punish Celes. In the original he simply says:
Kefka: I command you! Give me your power! Arrrgh!
Curses! Listen to me, or you'll regret it!
Give me... POWER!
In the more accurate version...
Kefka: Let me in here! Grrr
Now listen to me. No more playing games. I
command you... Show me your power
What's important here is that Kefka makes it obvious he wants to stand within the Warring Triad and later makes the famous linewhen Gesthal inquires about how Kefka is unharmed.
Kefka: "How" you ask? By standing in the center of
the Warring Triad, of course.... Or hadn't
you noticed?
Kefka seems to have positioned himself on purpose within the Warring Triad, and it also seems Kefka may know more about them than Gesthal seeing as the line that makes it look like Kefka figured it out on the spot is conveniently absent in the more accurate script. Let's not also forget the fact that Kefka begins the duel with a cocky laugh implying he knows something the Emperor doesn't:
Gesthal: What's so funny? Very well then... I guess its only
fitting you go out laughing.
I think the dialogue and Kefka's actions early in the scene shows that he may have been planning to take over at some point. A wonky translation makes the scene look like Kefka stumbled upon the truth of the Warring Triad, but the more accurate script shows that Kefka's silly action were intentional for victory, seeing as he makes a bigger fuss about standing in the middle of the Warring Triad. Kefka may have been buying time to get strong enough to take over. Cid's dialogue in the Magitech Research Facility also hints that Kefka may have been powering himself up on the side.
I do feel Kefka may have been using the Emperor (and the Empire) to gain power for himself. With these changes to the script, I don't think its unlikely Kefka wasn't manipulating everyone to gain power and the Warring Triad.