a pert and polish enlarges the intake and exhaust ports on the cylinder head to flow more air and gain more power. if you remove to much material though, you will lose power. i is hard to say what kind of power you can make, because of how good the job is done. you could gain anywhere from 0 hp-25hp.
Your also gonna gain more horsepower if your running forced induction or a different cam. The most important part of the engine when it comes to performance is getting air into the engine. The cylinder head is probably the most important thing to look at when building a motor. Any mods that you put on the motor will be enhanced if you put a port and polished head on there. This will allow the engine to get more air, which is the biggest obstacle in performance. A good port and polish should also have valve job that comes with it, which is gonna give you bigger valves and maybe stronger springs and retainers. Then you can run a more agressive cam, which will allow even more air into the engine.
I think they start out around $400 and go to about $1000 for a really good job. I've never had one done before, so the prices might be wrong. I saw a valve job and P&P for like $1000 on the internet somewhere. It isn't cheap, but if you really wanna get big power out of your motor, this is a must.
Is shiming the heads and milling the heads the same? If so, when you mill the head, you cut (or machine) the combustion chamber to be smaller. This is gonna give you a higher compression ratio.
the difference between shims and milling is that shims are little pieces that go before the valve and adjust clearance... Milling is grinding down the whole bottom of the cylinder head to its completly flat again and non of it is like warped or bumpy from like exsessive heat were it melts, or from boost which lifts the head and warps it
Port & Polish
