The only issue I'd have with that Yora is that because it's unpredictable, it'd be frustrating. Too often it'd feel like basically the DM waving his hand on if something terribad happens or not. I mean the beauty of Dark Heresy's system for Psychic Powers is that you KNOW when you rolled you messed up. "Oh crap, a 9. And I didn't roll enough to manifest my power but something bad is going to happen anyway!"

Whereas the Residual Effect system would play out more like this:

Player: I cast a fireball at the enemy horde. *rolls dice* Successful!

DM: Due to residual fire mana in the area the fireball leaves a wake of fire in it's path that spreads outwards and deals fire damage to all your teammates.

That's the sort of thing that makes me think would happen around the table. And it makes the game feel like the DM is just railroading you into a No-Win Fight (Or easy Fight if it works for you), or just is out to screw with you unjustly and with no real warning.