Polishing a plastic reflector with a reflective coating on it will destroy the surface, and then you have to send it out to be recoated with aluminum, using vacuum deposition equipment -- figure at least $50. Aluminum can be polished, and it can also be overcoated with silicon dioxide or othe protective coating. Do a search on 'telescope mirror coatings' and you can learn more.
While the optics in a flashlight is not 'precision' you don't want to throw them off too much or end up with a bad surface. There is all sorts of information about polishing optics you can find by searching about telescope making. I doubt you could improve the reflector much over the factory supplied surface, though, unless it was really bad.
If the surface of aluminum has deteriorated to where polishing would help, you could use red jewelers rouge (very fine red iron oxide), but don't use anything at all gritty or coarse. A bit of that on a soft cloth, with some water and a little soap would do, using your finger (or just your finger -- which will get a bit stained). There are other things you could such as making up a wax or pitch lap, and other optical techniques, but that get's more involved and you should look up those techniques used for optical work.
For all the work and such, I'd say it better to just buy a new reflector if needed.
Throw is not primarily due to the polish, but the shape of the reflector, or where the bulb sits in relation to it's focal point -- and you aren't going to change that by polishing.
