To answer the main question, yes, the grip does affect the flight and the movement of any breaking pitch. The more you have
If you're not slowing down your arm speed compared to your fastball (which would explain the loopiness) then there's not much you can do I guess. A power curve is a curve thrown as hard as possible. The harder the velocity, the later the spin will bite and the ball start dropping.
About the grip, I posted a post few days ago with a pic of a grip while trying to explain my own little gyroslider. Using the same grip on your curveball might help you variate it. You might consider trying it out. It's the forst post on the page...
I generally advice just to play around with the grips. The more you try different things the more you learn about yourself and what certain thing that specific grip does to the ball.