Shaving your cylinder head, effectively reduces the combustion chamber volume, and this inturn increases your compression ratio. There are general rules of thumb for gains from compression ratio increases, but simply put its difficult to make an accurate call because you are changing the combustion chamber diminsions/configuration and this changes how effective your mixture combusts, and how the flame front propogates, etc, etc. Very involved. . .
Don't quote me on this, but for some reason 5-10% HP increase per "point" increase on the compression ratio comes to mind. ie if you have the 2.0L which runs a 9.0:1 CR and puts about 100 horses to the ground, and you bump the CR to 10:1 you would see around 105-110 horses at the wheels.