Had a head worked for a customer, the work was done by a professional shop who employed a british gentleman who knew his zetecs. he did all of the work on the head, I told him the cars intended use and basically let him work his magic.
Very happy with his work, though his work is top notch so can be the price tag.
I wouldn't recommend doing it yourself unless you were versed in this kind of labor, porting is a dieing art form. Get a head from an auto recycler and practice on that first.
You can expect very high gains and better power curve depending on how good a job they do. A good professional job should run you about $500 -$600. I had mine done about a month ago and am very happy. It sounds awesome and really hauls A$$. Go for it