I lapped my ihs and my BT last night.
I havent noticed a huge reduction in temp, but I have noticed that my cpu cools a lot quicker after being at load. (may have to reapply the as5....because of the shiny surfaces they were slipping against each other when I was mounting the BT...not sure how well covered the core is)
I followed the guide that was linked to above for lapping the ihs....came up pretty well.....I was surprised at how curved the surface of the ihs was......the edges had worn to copper long before the centre did but now it seems to be all nice, flat and shiny.
the BT was so easy to lap.....because of the design of it's heat pipes you can place your hand on top of the base and keep it nice and flat to the sandpaper....they say with hsf to go back and forth rather than circles to prevent uneven wearing....this wasnt a problem with the BT...it was just like lapping my ihs...just take it slowly.
EDIT: I'm also going to mod my BT using the copper RAM sinks as shown in here