With boxes I mean the ensembles. You don't get pieces out of those, it adds straight to your wardrobe. The ensemble does display two distinct messages (which I missed completely): "You know some of the appearances granted by this set already" (in blue) if you already have some of the armor and "Already known" (in red) if you have all of it. If you know nothing, there is no tooltip.
You don't get warnings on gear itself unless you use an addon, afaik.
I'm not sure what you mean here.Could it be that the armour set transmogs you buy are because of that, and so you'd still need to buy them anyway?
Anyway, I bought the mail ensemble on my hunter, but since all the mail gear drops for me from the event, that was a waste. I should have bought ensembles of armor types that I won't get in the invasions (I can't be bothered to do them with all my alts). Ah well, I'll just need two extra invasions to make up for the 200 nethershards, so it's not that bad.
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Yet it is still easier, and faster, with higher ilevel. Significantly so when you go from 640 to 700.