I could recommend a specific cylinder head porting company if I knew one thing...
1. I know what your talking about, but many people here do not... porting can mean port polishing, or increasing displacement... I believe your talking about polishing considering increasing displacement will kill your gas milage and turn your VW into a 1/4 miler only.
I am going to assume that you are speaking of a VW engine because you have posted this question on my VW Forum...
http://www.sa-motorsports.com/diyport.htm visit this website and decide if this process is truely worth all the money youll spend. This site will also tell you how to do it yourself, assuming your good at this kind of stuff and you have an incredibly clean garage.