There was a thread on here sometime ago regarding the Mosquito bomb lifting capability and the reasons why they didnt build more.
The basic reason is that the Mosquito was mainly made in small workshops dotted about the country and not concentrated in one huge factory. It was a specliaist wood-working job. Also the demands for the aircraft , once its potential had been revealed was such that supply struggled to keep up!
Also British industry was 'geared up' to build heavies. The changing of production was not a simple task in wartime.
50MPH was the estimated increase a heavy could expect in added speed devoid of armament and gunners. However the Germans used JU88's with Nitrious Oxide to catch Mosquito's , so its probable they would have reacted fairly quickly when faced with a fast unarmed 'Heavy'.