what's a valve job? and what will it do for me?

 

It usually means a lash adjustment. The valves get tight over time and need to be adjusted back to spec. Loose valves tick, and tight valves make starting the bike difficult sometimes.

If you hear a loud ticking sound coming from the head that gets faster and louder with throttle, your valves are loose. If your bike has spark, good fuel and a clean filter but won't start the intake valve most often is tight.

 

Around my neck of the woods a valve job is when you grind the valve and seat to a perfect finish and match.
If you get a 3 angle job it will help the flow because you can set the surface mating size. In theory, the narrower the mating surface the more flow you will get. The valve will disapate less heat though.
Maybe a lot, maybe nothing.

 

I/we where I come from call that "cutting seats". Usually replace the valves depending on the wear and if they are coated or not. Do a 3 angle cut on the seats and lap them if neccessary.