That seems valid/good, yeah.
The primary consideration is that this also lets you steal the planeswalker's ultimate, and/or kill the planeswalker by using a sufficiently high negative.
For example, I cast
Nissa of Shadowed Boughs and use her +1, then pass the turn to you. You'd normally have to burn my Nissa or fight it in combat or outright destroy it to prevent me from accessing the ultimate, but instead for 3 mana you get to grab it, kill it, and use the ultimate yourself. I wouldn't feel great about playing a planeswalker with this in the environment.
My suggestion would be something along these lines:
Uber Threaten 1RR
Kicker B
Choose target creature. If this spell was kicked, choose target creature or planeswalker instead. Gain control of it until end of turn. Untap it. It gains haste until end of turn. If it's a planeswalker, remove two loyalty counters from it.
Now you can still neg it, and possibly kill it right away, but you don't get to ult it unless the opponent's just been ratcheting past the ultimate and ignoring it.