Souless,,
I've don't recall seen anything about Merlin 25s ever running at +16lbs boost. They are, as I recall, +18lbs boost or +23lbs boost on 150 octane. It has been a bit since I intesively studied this stuff though and I could be mixing up the Merlin 25 and Merlin 66 boost setting on 100 octane. This has nothing to do with 250 or 500lb bombs or flame dampers.
Only the early Mosquito VIs were limited to 250lb bombs and those, so far as I can tell, all had Merlin 21s or 23s and started the order process as B.Mk IVs or F.Mk IIs.
As to the flame dampers, after the initial runs they stopped putting flame dampers on as factory standard, but they were still put on Mosquito Mk VIs that were intended to operate at night. About 1/3rd had the dampers and the 2/3rd majority did not.
Bozon,
Right at the start. Some VIs came off the lines with Merlin 21s, some with Merlin 23s and some with Merlin 25s from the very beginning of production.