The RAF didn't think that the Mosquito was particularly good as a day fighter.
The AFDU did tactical trials of the F/B Mk VI in Mar-1943. The aircraft was fitted with Merlin 23 engines, allowing +14 lbs in combat boost, so was a little down on the speed and climb that can be expected of a Merlin 25 equipped example, but the results generally stand.
The aircraft was found to be nice to fly, with well balanced controls and effective ailerons. However, the inertial weight on the controls meant that "forces over 3G are very difficult to impose and detract from the general manoeuvrability as a fighter". It was found to be tail heavy in a dive.
Elevator control at 450 mph IAS was "very heavy" but aileron control was "quite light".
The aircraft was flown against the Spitfire V, IX and XII and the Typhoon IB.
Low altitude
It was found to be outpaced at low altitude by all fighters except the Spitfire Mk V. All S/E fighters were able to throw the Mosquito off their tails and get onto its tail themselves. The Mosquito was found to be unable to disengage whenever a S/E fighter got into position behind it.
However, its evasion was considered "particularly good", making it difficult for fighters to get easy shooting solutions. The aircraft was easy to corkscrew and could weave easily, even at high speeds.
The exception to being out-turned and out-manoeuvred was against a Typhoon flown by "an inexperienced pilot". However, "when the Typhoon, which has a turning circle similar to the FW 190, was well flown, it could make matters almost as difficult for the Mosquito as the Spitfire".
Medium altitude:
Similar results to low altitude tests. There was a better chance for the Mosquito to escape though. It accelerated in a slight dive faster than the S/E fighters. Time to max speed from fast cruise was two minutes. The Mossie could lead the fighters on a stern chase and sometimes avoid combat outright using a slight dive. Best speed was at about 9,000 ft.
High altitude
The manoeuvrability of the Mossie was much diminished over 25,000 ft and aileron control "feels comparatively mushy".
Enemy fighters
It was also flown against a captured 190A (probably an A3) and a 109G2.
The aircraft was found to be around about as fast as the enemy fighters near sea level, faster than both aircraft at 9,000 ft but slower at altitude, particularly against the 109 which was much superior over 17,000 ft. The Mossie was 700 ft/sec slower in initial climb than the 190A, and 1000 ft/sec slower than the 109G2 in climb. This deficiency increased with altitude.
Overall assessment
It was judged that the Mossie was not a good day fighter against S/E types. If in contact with enemy fighters it was "unable to go on the offensive and must content itself with defensive tactics".
It was not judged to be expected to behave as a fighter against enemy S/E types.
It was considered "only able to be offensive against enemy bomber type aircraft". It "cannot take on enemy single seater fighters effectively".
It was also considered to be a poor aircraft for bomber escort and would "probably be a liability to a bomber force".
Single seater
Interestingly, the AFDU converted their Mossie to a single seater and took out about 1,500 lbs of equipment from it (dropping all up weight to 18,800 lbs) and removed the inertia weight from the controls. The results improved the climb and manoeuvrability of the aircraft. Rate of climb shot up to over 3,000 ft/min for the first few thousand feet.
The consideration was that while the aircraft was "unable to be really effective against a Spitfire" in slow turns a lighter smaller version of the Mossie (ie the Hornet) could probably out-turn the Typhoon and "certainly" out-climb it. It was considered that it could "probably deal well with the typhoon of FW 190 class of fighter, especially by out-climbing them."
With Merlin 25s:
The ADFU later tested a Mk VI with Merlin 25s against a Spitfire XII. It was found that the aircraft could outrun the Mk XII at ground level. Even if the Mk XII was 800 yards behind and 1,000 ft above, the Merlin 25 outfitted Mosquito VI could accelerate from a fast cruise and pull away.
The RoC was also significantly improved, starting at 3,800 ft/min. The aircraft could go from fast cruise at sea-level into a 2,000 ft cloud base in about 30 seconds.
It was considered able to accelerate away from the majority of attackers. From a fast cruise top speed was hit in about 1 1/2 minutes (about 30 seconds better than with Merlin 23s).