Craig--I used a 9L bag of the PowerSand (not to confuse you, as it is quite chunky and not "sandy" at all). I used it in conjoint with AquaSoil II for my new tank. I put in on top of the kitty litter and beneath the AquaSoil; I also have mineralized soil on top of that and regular sand.
I believe that the powders that you see certainly are not going to hurt you. I didn't get them because I knew that I was going to have plenty of iron in my substrate, and just wanted the PowerSand in order to quickly establish bacterial colonies with the substrate.
If you are going to use a mix of substrates, you are currently dosing ferts to your water column, and you have a way to establish bacterial colonies in your substrate quickly, then you probably do not need them. I have no "cons" per se to mention other that they are pricey. However, I have heard that the additives do a great job. I am pretty neutral on them; I just think that there are cheaper ways to accomplish the same things.
If it is ADA, you know that it may be worth it, though!