I knew I'd seen the "speed reduction with drop tanks on = 5 mph" data somewhere. Found it on Mike Williams most excellent site:
http://www.wwiiaircraftperformance.org
Sharp & Bowyer give a 5 mph loss for the wing bomb carriers, which needs to be taken into account when looking at the speed quoted in the Ministry of Aircraft Production data card for the Merlin 25 Mosquito, which I've posted on the AHwiki.
Can provide page number reference for the Sharp & Bowyer quote at some point.
I'll use this info and put together a "composite curve" based on the speed gain from dropping tanks and from using the stub exhausts.
As for the dead horse, heheheheh, I've used the phrase meself, but I imagine the AH folks have had other priorities for the past little while, also for the immediate future too.