Although it is not an expensive adventure, the gains are all but non existent. Couple that with the thermactor humps and the general constipation of the 289 head, and I think you would be wasting your time and money. Get some 65-67 heads and do a little mild port work yourself (like getting rid of the thermactor humps) and you will be much happier.
I think a machine shop will generally take off 10 thousandths or less. If the heads have been decked before they should just do a minimal clean up resurface.
Decking the heads is one of those small details that goes with a lot of other small details that can all add up. Just increasing the compression by itself may not help though. It depends what your cam spec calls for.