The mast is strong enough, but not stiff enough. Upwind sails (and fast boats sail near upwind apparent) regardless of the true wind direction need huge luff tension to set well. This is impossible on an unstayed mast. It is also difficult to sheet them when they are tacked to the lee hull. Shunting is a pain; they have to be moved from end to end. Symmetry doesn't work as the boom has to rotate through the screecher.
These are the obvious reasons why harrys don't carry screechers, but there are less obvious ones as well.
The weight of the screecher, the extra crew, the sheet, halyard, furler, winches and the beefing up all these require have to be carried upwind. On a light boat, this is a significant portion of the all up weight. Including the extra weight to make the boat strong enough, it is not far off 50% using your numbers. The lighter the boat, the higher the power to weight ratio and the less a screecher is required. I am not sure when the trade off makes the non screecher boat faster all round but it definitely exists. A Bucket List built to the same spec as the M32 (all carbon) would give the M32 a run for it's money around a short course. In terms of cost, maintenance and ease of sailing the non screecher boat is a no brainer.