Head Skim
 

I wish to retain the standard pistons, but I am gonna gas flow the heads. What would be the safe ammount to skim off the heads ? :D

you can do a search and probably find it, I believe a couple have done it here. Otherwise those that have will chime in. It will alter the cam timing slightly but as you are doing all this work you were probably going to degree the cams anyway, just a reminder to be sure to do so.

Chap who did my heads took 0.015" off and it runs fine on 87 octane (he also cleaned up the combustion chambers to remove hot spots).

Is the performance increase appreciable?

Hard to say what did what as I had a few things done at the same time: heads ported and decked, along with installation of Yosh Stg 1 cams. With that in mind, it ran noticeably stronger afterwards :)

Thanks for the help, 0.015" sounds good to me !