From a balance perspective, I think it is reasonable to rule that studded leather is metal armor (at least for this power). The power does 1D6+1 damage per PP for fire and cold, but 1D6 for Lightning, just in order to get +3 to hit. Since it is a range touch attack, it will hit most of the time anyway, so the manifester is giving up 1 point of damage per PP in order to get +3 to hit.
Ranged touch attacks hit at least 50% of the time (and often a lot more often) for most characters against most similar CR level opponents. Hence, 50% * 4.5 per die = 2.25 per die versus 65% * 3.5 per die = 2.275 per die means that the manifester is often averaging about the same damage for the attack by using the Lightning. And as the chance to hit increases (as levels go up), the Lightning option averages even less damage per die in comparison.
Pros and cons, it seems balanced (at least in this case).