With the type of corals you're planning to keep, I'd say that your two powerheads is fine in terms of flow.
I honestly don't know what you mean by overkill... A canister filter is packed with filter floss and carbon and etc. But the main 'rating' that is given to those things is how well it handles the bioload of a freshwater tank. If you do buy that thing, you're going to have to take out of the floss every week and clean it because of the build up of nitrates.... I don't know why anyone would get one of those things for a saltwater tank... unless you want to hide a heater and add little bit of flow.
Also, spray bars dont add direct flow to your tank. all they do is add surface agitation.
If I were you, I'd skip the canister filter and either buy a small fuge or a skimmer.