valve clearance dont affect performance in that way
the idea is that you leave clearance when cold.
when the engine heats up, and it all expands, there will be ALMOST no clearance.
the large clearance is due to manufacturing tolerances. a blueprinted motor can be made tighter.
the valve closes completely and seals the port just as effectively when turned over by hand as when redlining at 12000rpm. except when you get valve bounce, which is when the spring simply cant close the valve fast enough... and using harder springs takes more power and increases bearing wear etc...
having a very tight clearance allows the valve to open MORE, as theres less slack for the rocker/cam to close before operating the valve. this allows more gas through. be it exhaust or intake. this means better volumetric efficiency. which means more power
the con of a tight clearance is when the valve ROTATES. if they arent perfectly true, the clearance changes. so suddenly there maynt be any at all!
this means the valve doesnt close, means no compression means no go
the valve also requires to be closed for a certain amount of time to cool down or it will melt
overdo it and you burn valves and seats...
BUT!
having too much clearance does the reverse regards efficiency. less air means less power.
and as the rockers/cams take up the slack, they hit the head of the valve with more force. keep hammering at the valve stem like that and it will mushroom over, clearance will increase, hammer effect gets worse, stem mushrooms more, clearance gets bigger, hammering gets worse,.....
ive always used .004. standard sorta figure across most manufacturers...some differ, but usually... 0.004
and make sure youre checking at tdc everytime!