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Wing Commander Warburton may have been a 'one off' but made aggressive use of the Twin Wasp Maryland destroying 8 aircraft so a faster better armed R2600 Maryland has to have some potential.
However the A-20 had excellent aerial performance for a 1941 bomber. Once the bombs were gone many contemporary (i.e. 1941) fighter aircraft would have a tough time catching it.
The Martin Maryland bomber was fast for it's day on fairly low powered engines, with forward firing armament, reasonable range and was able to perform low level unescorted PR missions and let it's pilot shoot down the opposition.
How far could the design be taken without making it something else?
Martin went down the road of a better medium bomber as requested by Britain as the Baltimore. Maybe the 60% power increase could have gone down the road of increased speed and climb and/or carrying more fuel for range.
This issue came up in an alternative Mosquito thread but I see it as a better Beaufighter.