Obviously a lot has to do with the bike and system, but overall, a slowing of the base timing will yield less kickback and prone to stalling, a smoother take off and associated reduction in top end performance. Forget trying to measure mm BTDC, it normally takes about 1mm rotation in advance rotation on the stator plate to make a change from stock, and about 2 mm on the retard side to make a difference. Any change here is setby feel of pants to pref, yet as a rule I would limit any advance to a mm or two, and decline to 5-6 mm, as anything out of this range gets too far out and other problems can occur.
Obviously this is a very generalized statement, but the basics apply!