A 150W HPS provides about 17000 lumens of light. Cannabis needs about 4000-6000 lumens per square foot when flowering to thrive so you're a bit under the optimum (wait you were the guy with fluoros too? again: keep 'em there) but close enough to surely get a good result.
The light that a 150W HPS provides isn't very penetrative (55cm/~1.8feet) and part of that gets lost in the space between the light and plants so what you need/want is an even canopy that covers the entire area to maximize the usage of the light. In other words this means that you need to use some technique like SCROGging or LSTing. I'm sure there's more info about those available in the GrowFAQ section..
About the home made reflector... difficult to say if it's good or not, depends a lot on the shape of the cooking dish, but the kind of reflector you want is something that distributes the light over the entire area. A commercial (but a bit expensive) option would be the Adjust-A-Wing. White paint for the walls is fine, no problem there.