I don't know about the Spits engines exhaust but the Rolls Royce Merlin Mk. 102 which had straight-out exhausts (versus the shrouded exhausts of the Mk 85) were exceedingly painful to the ears of crews of the Lincoln Mk 31 - Long Nose. With a take off power of +12 boost and 3000 RPM increasing to +18 in an emergency, and the inboard engine exhausts barely 15 feet from the cockpit, it was almost impossible to talk on the oxy mask intercomm system until power was reduced to climb boost of +9. Extra speed attained through these exhausts? I don't recall it.