Don't lap a heatsink with exposed heatpipes in the base! Those pipes are really thin and you could easily put a hole in them.
I lapped a hyper 212+ with no problems. I didn't check any temp difference as I lapped before installing. The gaps were noticeably smaller after lapping and it felt more smooth.
If your going to lap just lap in moderation. Just don't sit there lapping for hours until it breaks :P
Used artic-5 silver.
Results i5 email@example.com after 6 hours prime 95 100% load, 59C
1.26v in bios
cpu-z says 1.21
load-line disabled (lowered temps a good bit)