The transformation doesn't necessarily make the card more powerful, but it maintains Flash, adds Double Strike, meaning you double the damage output in combat, and shifts the card draw from opponents drawing two cards per turn to combat damage dealt to players
Jace, the Mind Sculptor was the one and only time we had put four abilities on a single Planeswalker (okay, okay, Garruk Relentless/Garruk the Veil-Cursed had fourwell, five, actually, but that was over two sides)
It's only now that a god deck with a more restrictive color identity has come out that he's become indispensable
Players are better off dropping Szarekh entirely and letting Imotekh the Stormlord take the stage
If you do, create a token that's a copy of that creature, except it's 1/1 and has toxic 1