nah it's not extreme derfist, you just need patience..
go to www.easypckits.com
for some good low priced heatsink lapping kits. You lap it on a mirror for a flat surface and basically use the weight of the heatsink itself as pressure. The cheap kit goes up to 2000grit but the premium grit goes from 400,600,800,1000,45 micron, 25 micron, 20 micron, 15 micron and finally 10 micron, and includes 10,000 grit polish for the 'ultimate mirror finish' I don't know about 4 degrees just cause of lapping but that I suppose COULD happen but don't expect it. I lapped my heatsink, but I didn't have this particular heatsink installed beforehand so I don't know the previous temps. =/
It takes a good solid hours work if not longer than that.....wet sanding is best...you can never use too much water, but the guy at that site I linked has good instructions