Its clearly stated Kefka was one of the original test subjects but your suggestion does add weight to the idea that Kefka became a General/Court Mage after the procedure as opposed to testing it on a General.

Still, it should be noted most of the Imperial soldiers in Vector actually use magic so there may be a core unit that are Magitech Knights, while normal soldiers use Magitech vehicles like the Magitech Armor and Flying Armor. I'm sure there would probably be many test subjects but its a shame we don't learn of their fates or even whither they actually existed. That's just the problem with 16-bit games though.