How should I mount a TV to a wall that slants inwards 8 degrees?
Your approach sounds perfect to me.  3 tips:
1.  Err on the side of thicker plywood.  3/4" sanded pine is what I'd be thinking.  3/4 MDF would probably do fine as well.
2.  You can use pretty much any CAD program to tell you the correct angles/dimensions for the upper and lower wedge-shaped 2x4's.
3.  Upper wedge?  Yes, I'd cut a thin wedge of 2x4 to put at the top as well, so that the fasteners aren't trying to bend the top edge of the plywood as you draw them up tight.