It's less the exhaust guards as the saxophone exhausts beneath them.
Saxophone exhausts had two stubs at the front of a longer vessel, thusly:
They produced a good deal less exhaust thrust than stub ejectors, as well as (so I recall HoHun saying) producing some backpressure on the engine.
I may be taking HoHun's name in the case of the latter statement, apologies if so.
The saxophone exhausts took around 15 mph off speed, the drop tanks a further 5 mph (going from memory on the tanks).
The two key quotes from Neil's site (in terms of the Mossie) are:
"It will be seen that at constant boost the effect on true air speed of fitting the multi stub exhaust system is a gain of 15 mph in MS supercharger gear and 13 mph in FS supercharger, "
and
"Comparison of results @ +18 lb/sq.in. boost with those of Mosquito VI HJ.679, indicate that the speed of that aircraft is of the order of 10-15 mph below average, thus supporting the conclusion made in the 7th part of report No. A. & A.E.E./767,e, that HJ.679 was not fully representative of the type. "
Sadly, HJ679 is the aircraft which AH appears to model.
