I can't speak to Jandice, haven't played with her enough. This seems aimed more at rogue and shadowstep than mages. One more mana can be a pretty big difference for rogues.
I had the same thought initially on DoL. But 4 mana might be enough. Pay 2 mana of tempo on turns 1-3 is not that costly. A lot of control deck hero power pass in the early game. 4 mana of tempo later in the game is much bigger than just 2 more mana of tempo. Your opponent's board is more developed, an aggro deck is close to killing you if they've had a good draw. The next spell you draw will, on average, not pay for the tempo loss. So, you're effectively two turns at a tempo deficit just to play that card.
My gut feel is that RSW with C'Thun was taking advantage of an underdeveloped meta similar to contol warlock. The typical shell for spell mage doesn't have the longevity or card draw to last getting through most of your deck by turn 10. If Flame Ward and Reno cards were still standard, maybe that would be otherwise. I'm betting Mask of C'Thun and burning your opponent's face will be better.