There are many levels of port and polish and different types. The two types are CNC and hand ported. CNC is a computer process which removes aluminum from your head based on computer software and the results depend on how good the program is...some do this better than others. Hand porting is just like it sounds, all done by hand with a die grinder with various cutters to remove material.
As far as the levels, you can get what many vendors refer to as Stage1, Stage2, or Stage3. Stage1 typically is gasket port matching, Stage2 typically is gasket matching with bowl work, and Stage3 is gasket matching, bowl work, and runner work. The biggest improvement in my opinion is the bowl work.
Horsepower levels will vary depending on what level and type of port and polish you go with as well as cam selection. Remember...it's always OK to go to small on the cam. If you go to large...you may have a race only car! Just depends what you are looking for.