many harley performance builds aim for 0.030".

Go for your life. I didn't Google any links, just have pulled a few top ends off with witness marks on the head/ piston. I do qualify that by saying I don't know about the EVO clearance, just twin cams. 

Performance build implies running a good tune, or carbon build-up will close up that gap and lead to piston/head contact.

Mate I spend a fair portion of my life tuning (and fixing)Harley "performance builds" almost no-one checks the squish properly, maybe in the world of the interweb, but not in real life. And I do mean almost no-one.  One after another they come in the door with less than optimal settings or builds. For a STREET bike, not a drag bike or a coffee shop bike, they need to be safe and consistent with a good tune, and a good combo of parts. HP and Tq will also come with those ingredients. I set all the harley stuff at .040" because of the above reasons. If I was building one for myself I might sneak it to .037" but really the difference in real word performance in say a 130hp TC110 with a .040" squish to the same engine with a .035" is almost not measurable. At .030" one is relying on the cylinder growth from hot to cold to keep you clear from contact, you personally may get away with it but the ones I see don't. The current world of M8 web builds is to fit a miracle cam and have mental cylinder pressure, you get staggering torque at WOT at 2000rpm  (where you never get the chance to do that on the road) and a great dyno sheet for the front bar, but in real life( not a four second dyno test with the knock sensor turned off) the poor thing is baking hot after 20 minutes riding and then pulling 5 degrees of ignition out on the knock sensors anyway. Where is the torque/power then?

Wombat, if you have the thing apart and then measure the squish(piston to head clearance), I am suggesting maybe .040" or 1mm is a good option. As Hoody said previous most EVOs are way more than this. Fitting thinner gaskets or light machining will get you a significant gain going from say .065" to .040" . The gain you will get going and extra .005" closer will be negligible. It is a 35+yr old bike not a MotoGP engine.

You mentioned additional mods in the future after fitting a cam now, that's how it started to get off track.

I recon we can milk this thread for another week or so!  lol

Personally speaking I have been enjoying the technical aspects of this thread. I would be enjoying it even more if my head wasn't bursting !

Not that I am confused or anything but what is the difference between compression and squish again?

What blew me away was just how high the compression was when I was running at 10lbs of boost!  Thank god the starting compression is a mighty low 8:1 on Evos...