The one criterion for Chang's torps was that they need to look exactly like Starfleet torpedoes in action. And not any generic Starfleet torpedoes, but Starfleet torpedoes set to a yield that will knock out gravity aboard Kronos 1 without destroying the Klingon ship outright.
It appears that Chang knew that Cartwright was in on the conspiracy and vice versa (and Cartwright's cohorts seemed to know about the Romulans, too, although we don't know if Chang's did). Clearly, the two sides thought that they could and should cooperate in a plot that would launch a war in which they could then gain the upper hand from their opponent. So it makes sense that Cartwright would sell Chang the genuine-looking Starfleet weapons, while making sure they were no better than they absolutely needed to be, ie. the low yield setting only...
Basically, then, Chang was an assassin armed with a Derringer, and needed to use the only weapon in his disposal in order to bring down the assault rifle -toting, flak jacket -donning Kirk. Obviously, it would be slow work.
...But if Kirk realized that, he'd gain the upper hand pretty quickly. So Chang cleverly put on a madman act, pretending to go for a slow kill because he "was a sadist"!
The torpedoes from Chang's ship was comparable to the Reliant's torpedoes striking the Enterprise in TWoK.
But Khan was firing to wound, not to kill. So his torps would be low yield, too. Except in the nebula, but there he failed to score any hits.
I just think it was easier to fire torpedoes whilst cloaked compared in pure power consumption reasons and that compared to using disruptors which would not look right on screen.
I don't know about looks, and the "disguising the launch point" thing was dependent on Kirk's reaction time which was so slow that it should have allowed for disruptor use, too.
But I can buy the energy requirements argument; a loaded torp might require
and emit fairly little energy, whereas active disruptors let alone active shields would glow through the cloak all too easily, making the firing cycle agonizingly slow: decloak, turn on guns, wait for guns to heat up, fire guns, cloak again, as opposed to the decloak, fire torp, cloak again that we saw happen (in practice, that is; firing the torp resulted in a light show that was as good as brief decloaking). The alternative would be to have ineffective cloaking all the time, due to the hot and shining dirsuptor coils.
Timo Saloniemi