Compression stop valve in gravity fed cold water pipe.

In order to help future servicing am thinking of putting a compression stop valve into a cold water supply pipe feeding a gravity fed mixer shower. I have noticed that even when the valve is fully open there is a fair restriction from a 15mm diameter down to i would estimate about 8mm. Will this affect the performance of the shower due to a restricted cold water flow? I am already working with a minimum head clearance so don't want to reduce to a dribble. I have used these things before but only on a mains fed pipe.