Not from what I have seen. Some people do them before xbows and wiz towers even. But they are pretty much always done before archer towers and cannons, some people do mortars before but mortars are useless so theres no need for that.
But to the OP, as you are probably aware by now your model is too simple against loons, and you are more likely to be hitting hounds than loons anyway, where the pure dps comes in use.
In the end air defence is your best defence against air, rather than your comparison, if you were to ask yourself how many other upgrades you would have to do to compensate for not doing that upgrade it would become clear that its worth it. No use saying it takes 'only' and extra shot when to compensate for it you would need to max two archer towers, costing 28mil and 26 days.

