But about that Mirari thing. Note that when you play a spell, the ability goes on the stack. Not until it resolves do you decide whether to pay the 3 and put a copy of the played spell on the stack. This will work against jumpy and inattentive players, who will foolishly jump the gun and waste precious counters.
Mirari is no more abuseable that Magistrate's Scepter, which so many were freaking out about a couple of years ago. There could be some interesting buyback interactions, but that's extended and I can't see extended being a good place to spend that kind of mana.
Another rules note: The spell is an exact copy of the original, no matter how you paid for it. You can play Gush and other ACC spells with their alternate costs and still pay only 3 to make a copy. You don't have to repay the ACC. I'm not certain at this point, however, whether paid kicker costs will be considered paid for the copy. My gut feeling is that they would be, but I'm not really familiar with the new copy card rules.
Kamahl is one of those rares you hope for in limited. Never a Ball Lightning, he's still 3 points of damage right away, and 6 if there's nothing between him and your opponent. You could squeeze him into a deck with plenty of removal to clear his path, but for 6 mana wouldn't
you rather have something a little more durable?
See the topic line for my thoughts on the flashback Boomerang. I don't expect to ever see this and the oddly-named "Smash to Bits" in a constructed deck, except
maybe Odyssey-only constructed. I'll withhold judgment against them in limited until I see what else there is to choose from. We may not have any other options.
I'm hoping we're not about to get stuck with a bunch of slow flashback sorcery-speed staple cards. I'm sick with worry over the though of paying 3W for a slow Disenchant (named "Put That Away?") even if I can play it later for 7WW.