Lumps, you are correct that the very coarse fixed pitch prop would hold back the RPM at low speed, and so reduce the supercharger output, but the Merlin was fitted with a boost controller which throttled the supercharger to prevent excessive manifold pressure. This could be overridden in emergency, with risk of engine damage/failure. Unfortunately, I do not know if the override completely removed control of boost or, if it allowed a higher controlled level of boost. The latter would seem more likely as the amount of extra boost available at low level was considerably more than rated boost.
Cheers