Most of my CL decks they I've been using have 2-4 shards. (usually 2 shard of focus :p) Other than my monodeath poison deck and my shard golem deck, none of them have more than 5 shards. Most of my toughest matches in CL have been against other people also using shard of focus. It's definitely shaping the metagame there a lot. Giving it an activation cost would help some, but in rainbow supernova/quantum tower decks, it'll barely be noticed.
A different way of adding another activation cost to help balance it would be to add a cost that was something like 'discard a card from hand', 'discard a card from hand at random', 'sacrifice a card in play', 'sacrifice a card in play at random', (or limit it to either creatures or permanents), or allow your opponent to get a benefit, like destroying one of your cards back, drawing a card, gaining life, or costing life yourself. (though an HP cost would be negligible, unless it were quite high; something like 5 hp/use would not affect anything, while 10, 12, or 15 would)