I really, really like EBS. They've been awesome to work with, and I'm sure whatever you have done there will be quality work. The know Porsche engines very well! I don't have any experience with any performance improvments they do, though.
All that being said, I believe there may be others that can possibly squeeze more out of head work. Anyone who knows these engines, four or six will tell you the money is in the heads. I saw a presentation given by Leo Goff of Memphis Motorwerks, and this guy is the real deal. I had a 3.0 built by Pat Williams in Memphis, and Leo did the heads on that engine, and the man knows his stuff.
Let's face it, anyone who knows their stuff can get flow at full valve lift. It's less than full lift where the real gains can be made, and that's where Leo has done extensive testing and improvements. There's a reason why people all over the world are shipping heads to Memphis to have them done.
There are lots of competent head rebuilders out there, and there are also plenty of crooks. Definately go with one who has built a reputation.
Just my .02
PK