I wouldn't bother with all that expensive coil over stuff and then half ass the camber plates....also the coilovers put a LOT of stress on those plates....much more than you have with a typical suspension setup, so the added strength of billet is manditory if you ask me, otherwise you probably risk some damage and/or maybe wrecking if you hit a good pothole or something. I could see the stock plates give out..
I'd have to say get some good caster camber plates. Slotting them will just weaken the stock ones, plus you don't know if you'll be able to slot them enough to get the proper alignment.
I thought of a better way to word this....coil overs focus all the suspension on the caster/camber plates and the shock towers themselves. Most people are conscerned about possible damage to the shock towers themselves (Griggs and i think Maximum Motorsports both have a front suspension setup that uses something like a double A arm setup that eliminates the use of the shock towers all together). If you were to use just the stock plates, and then slot them, you're weakening a little 1/8th inch sheet of metal that was never intended to soak up all the force of the front wheels, since in stock form the springs were somewhere else and took up a portion of the stress while the shock towers and CC plates took the other half.