Here is my experience with leakdown and newly rebuilt heads -

http://forums.pelicanparts.com/porsche-911-technical-forum/1053367-chasing-leak-down-after-valve-job.html

I also had a very reputable Porsche engine builder do my heads. Mistakes happen and two of the exhaust valves were not cut all that great. The shop re-cut them.

The frustrating part came next. Lot's of advice that newly rebuilt heads should not leak at all. Not my experience. And torquing the heads down changes the seal as well (non leaking valves would leak and vice versa).

Fortunately after running the engine for a 1000k miles the valves all seated perfectly and my leakdown numbers are great <2%.

Hope that helps some, and best of luck!