I did this to my ST170, i imported a CFM pully amongst other things from the states. I retarded my Timing -8 as i found this at the time to be best for my engine. It equates to exactly half a tooth retarded so...............technically if your confident enough you could time your stadard exhaust cam to be half a tooth out, but i wouldnt reccomend it as -8 is getting close to valve contact.
It will not give you more power but it will move the power band up the rev range. If i did it again i would only go -6 degrees as at -8 it bogged down a bit on launch but as soon as you get up the rev range it would pull and pull and pull. I had a long blast of a few miles with a Merc once on a private track of course and i was pulling away with a GPS indicated 147mph. So it definately helps top end. It just makes the car more lively.
You dont need to map it, set it at -4, go drive it, then at -6, go drive it, then at -8 and see what suits you the best.