The thing with valves is that the valve clearance increases as the engine warms up, and the larger clearance means that the valves open later and the lift is smaller. Practicly you could set the clearance on cold engine to .0mm, and there would be no colision between the valve and the piston head. But you would need to recheck the clearance more often since the valve seats and the valve heads tend to wear even more if the lift is larger. Also, the mixture tumbles as it hits the valve head and doing so it homogenizes which causes better combustion. If the lift is too large there is no thumble. So if you want larger lift, you need valves with larger head diameter to achieve the tumbling. That's why the better way of setting valve lift and timing is to replace the camshaft or modify the cams on the existing.