I don't know much about the built in clearances of stock valves but wouldn't the stock valve train be designed to deal with the full amount of advance or retard available within the avcs system? Otherwise why not have hard limits set within software?

My reasoning for this is that often the tuner is not the same person who built an engine. I also recall reading a letter from Cosworth that suggested their big valve heads could accommodate the full avcs advance.

If not, then it would be easy for a tuner to clip valves on Pistons when tuning to the max.

Would love to hear from those that know.