I think the best bet would be to research the individual squadron histories and see if/when they report the modifications.
Yes indeed, though that said the best source for the timeline of on-squadron technical changes is actually Sharp Bowyer. The original ORBs tend to be full of who's joined the squadron, who's left the squadron, what the weather is and what exercises were undertaken. The post-war histories tend to have a lot of info on who dropped how many bombs where and when, though Barry Blunt's squadron histories tend to have much more info.
Anyway, in terms of what Sharp Bowyer have to say about speeds and tech development, I've gone through it again with a view to quotes about early speed tests, early engine management and early exhaust developments. It all goes to explaining why I believe the early bombers and PRs did 380 mph (my Inner Crumpp just won't let it go...)
In February '41, de Havilland's tests on W4050, the prototype were in line with expectations, which had been 386 mph. When the A&AEE tested the aircraft at Boscombe Down in March '41, they found the speed to be 388 at 22,000 ft. After it went back to de Havilland's, the latter were getting a top of 392 out of it, at 22,000 feet and 16,000 lbs. weight. (I'm assuming that was on +9 boost, as that seems to have been the FTH for that boost level.)
That's the prototype of course. The company delivered a total of 20 aircraft before the end of 1941, and "As performance was measured on more and more aircraft it appeared that production B.Mk.IVs at the end of 1941 were doing 382 m.p.h. at 22,000 feet, 5 m.p.h. less than had been measured on the prototype with faired nacelles. Fighters were doing 378 m.p.h. (352 m.p.h. with matt finish), the greater drag of their flat windscreen, plus A.I. and larger tailplane, being offset by their better nacelle fairings at that time - until the final nacelles became standard."
That speed appears to have been borne out once the aircraft got onto squadrons - the PRU was the first user, followed by 105 Squadron on bombers:
"105's first Mk.IV did not arrive until mid-May (mhuxt - 1942). With the longer nacelles and flame dampers these were reckoned to be faster than the P.R.U./Bomber Conversion Type Mosquito I, but on tests the first showed a top speed of 2 m.p.h. less, 380 m.p.h. at 22,000 feet in F.S. gear, and 105 Squadron maintained this was general."
There was also the issue of how much boost could be used, and when, but the evidence suggests that the emergency boost cut-out was useable from March '42 onwards at the latest, a couple months before the bombers went into action over Germany. The PR birds eventually encountered Luftwaffe fighters, as below:
One "of the earliest encounters with fighters (was) described by Flt. Lt. Merifield after a flight from Scotland:- 'We were flying in W4061 (mhuxt - 12th aircraft produced overall, seventh PR machine, 2nd of the Long-Range variants, would have been delivered with ducted exhausts, not the saxophones or the stubs) on 30 March, 1942, over Trondheim at 18,000 feet in F.S. gear at 2,400 revs. at the time. I noticed an Me 109 in my mirror about half a mile behind and 500 feet above. It was making a trail of black smoke, presumably because it was at full throttle. I increased revs. to 3,000, switched over to M.S., pulled the cut-out and dived gently. My observer reported another 109 on our starboard quarter about the same distance behind. We levelled off at 14,000 feet but did not seem to draw ahead. Observation of the enemy aircaft was difficult because they were dead astern and we were making a lot of black smoke ourselves. After a quarter of an hour they were no longer to be seen, so boost was reduced to 6 lb. and revs. to 2,700. ... I could not say what speed was reached during the dive but at 14,000 feet the I.A.S. was 320, which afterwards computed to be 395.'"
The important thing about the above is really that the boost cut-out was available, allowing higher boost below 22,000, and that pilots were ready willing and able to use it. This wasn't just the case at the PRU - the bomber pilots of 105 also used it in mid-42, about a month after their first raid:
"During his sortie (mhuxt - to Flensburg on 2 July 1942) Sqn. Ldr. Houlston was chased by three Fw 190s, intercepting Mosquitoes for the first time. He comfortably drew away from them, using plus 12 1/2 lb. boost at sea level."
Furthermore, the available boost levels were increased shortly thereafter; as noted earlier: "By the end of September (mhuxt - 1942) 105 Squadron had nineteen Mosquitoes, all with +14 lbs. boost."
As far as the stub exhausts are concerned, the prototype was first fitted with stubs in July 1941. The initial tests were done at Hatfield, but were adopted after trials in the operational units themselves. "Trials at Marham (mhuxt - 105 Squadron's base) with DK336 fitted with open stubs indicated that these gave too much glare at night, revealing the position of the aircraft and making night landings difficult. Closing the stub ends to oval section of slightly less area reduced the glare in tests on 25 November (mhuxt - 1942) and increased the jet propulsion effect without reducing engine efficiency, giving a worthwhile net gain of from 10 to 13 m.p.h. depending on altitude. ... The shrouded exhausts - never popular with ground crews - were retained for dusk and dawn attacks, and others were fitted with oval stubs for day raids, as on reconnaissance aircraft."
It seems the work at 105 was in fact applied on the recce aircraft either before the bombers got theirs, or at exactly the same time: "Two Mosquito IVs fitted with multiple ejector exhausts reached the squadron (mhuxt - by this time the PRU had been split into various squadrons, in this case 540 Squadron) in November (MH - 1942). It was decided that the extra 10 m.p.h. or so obtained from propulsion effect was worth more than flame suppression, and future aircraft were similarly equipped."
So anyway, that's where I'm coming from on early Mossie bomber speed. If the U.K. had gone to the U.S. with a plan for the Americans to produce Mosquitos, it would have been for a 380 mph machine.