I dislike "we built the deck for you" mechanics like tribal and this Cthulu thing. I hate it a little less than other we-built-it-for-yous though. It looks like there might be a little bit of flexibility.
Playing your huge Cthulu is the big Timmy thing everyone wants to do, but if the mechanic is good at all it is going to be about the "Do X if your Cthulu is sufficiently large" cards. I'm guessing the 3/4 will be unplayable because it's outshined by the even valued ones that get you to the clean 10 efficiently.
I do wonder about this a bit. Everyone will be playing C'Thun, because everyone will have one. We already know of four synergy cards, all neutral, right? So nine cards of your deck are set at that point.
I feel like Mirror Image on T9/10 from Mages will become incredibly common.
You only need enough of the synergy cards to get your Cthun to 10/10 reliably and activate the "10 or more" reward cards. It's extremely unlikely that you'll get Cthun to 10/10, put yourself in a position to play him, but then lose the game after playing him because he wasn't 12/12 or 14/14. So there should be some flex room to cut some of the Cthun synergy cards for Water Elemental or whatever thing you want your particular deck to have and still get to 10/10 ok and be good to go. Which is why it's not as worrisome and linear as Mechmage, hopefully.
On the other part, yeah you might be right, Mirror Entity's not a bad card, saving it for turn 9 doesn't seem awful if the matchup calls for it.