Graham: The extrapolation was very slight as the Mosquito FB.6 Pilot's Notes has its curves (at SL) cut off at 170 kts. TAS. At 170 kts. the fuel flow is about 66 g.p.h. Mosquito PN can be found here:
https://ww2aircraft.net/forum/thread...to-fb-6.22836/
As for the 2000 r.p.m. not being ideal, the same set of curves suggest that up to 220 knots is obtainable at 2000 r.p.m., thus at e.g. 160 kts. the power reduction is obtained through reducing the r.p.m. and this obviously violates the high boost/low r.p.m. rule. With standard Merlins the highish minimum recommended r.p.m. was the generator not charging properly at low r.p.m. According to David Birch of R-R Heritage Trust, with certain modifications, the Merlin could be run as slow as some 1200 r.p.m. The Allison could not this routinely. And in fact, e.g. the Bf 109 F-4 got its best air mileage at lower altitudes at about 1350 r.p.m.
BTW, e.g. the F6F Hellcat pilot manual recommends r.p.m. down to about 1300 for cruising for max. range and endurance (ma. range I.A.S. is given as 135 kts.).