Not entirely; the effect of synchronisation depended on a number of factors; the relationship between the guns natural RoF and the propeller rpm; the number of propeller blades; the quality of the synchronisation system and the RoF of the gun (the higher the RoF, the bigger the reduction).
In effect, a figure of around 10% would be about right for most WW2 installations, with 20% being the maximum reduction.
Tony Williams
New book: "Rapid Fire: The development of automatic cannon, heavy machine guns and their ammunition for armies, navies and air forces"
Details on my military gun and ammunition website:
http://www.users.globalnet.co.uk/~autogun/index.htm