I think Thunderstrike benefits greatly from casters with the natural levels for it. All those extra levels of air magic add a point of precision, which is invaluable. A4 or A5 Eagle Kings, for example, start with 16 and 17 precision, respectively, and with how quickly they build up stars, you quickly have a bunch of 20+ precision thunderstrike spammers.
Your Theurgs, meanwhile, start with a rather meager 11, and even your more experienced one will only be hitting 14 or 15
