The tables at AHT assume that all props have the efficiency of 80% - that would tend to skew results? Also, there is no exhaust thrust taken into account.
Most props are going to be within a few percent of 80% at sea level. At 20,000ft or so were the air is about 1/2 as dense prop size and design becomes more important.
Exhaust thrust doesn't quite equal power, it varies in efficiency with the speed (and altitude) of the aircraft. At 250mph and at sea level it contributes quite a bit less than it does at 20,000ft and 325-375mph.
The only figures I have are for a Hurricane II with a Merlin XX engine. Charge airflow though the engine at 15,00ft is 140.5lb/min and at 20,000ft it is 144lb/min which is fairly close (at 25,000ft it is 129lb/min) but due to lower atmospheric pressure at the exhaust tips the exhaust gas velocity changes from 1395fps to 1695fps with the change in altitude ( 25,000ft brings 1840fps) and there is only a 10mph difference in speed with 25,000ft splitting the difference. Exhaust
horsepower is 86.5 at 15,000ft, 113 at 20,000ft and 107 at 25,000ft.
IF the accompanying chart is correct and
if you continue the lines down to below 5,000ft it looks like the Merlin XX is making under 50hp in exhaust "power" below 5,000ft. Or a change from providing 10.5% of the prop power to 4% or under of the prop power at sea level.
Very few radial engines had exhaust stacks as well executed as a a V-12. Please also remember that exhaust thrust is dependent on exhaust mass ( fuel, air and
water/alcohol) early aircraft without water/alcohol/over rich mixtures and the like have less mass than later engines (or even two stage engines) , exhaust speed
and to turn into useful power , the aircraft speed. the closer the match of the exhaust speed to the aircraft speed the more useful the exhaust thrust becomes. One of the great advantages of the variable pitch prop. At low pitch (take-off) the prop may be trying to accelerate the air to around 1/2 the speed it would be if it was stuck in high pitch. (Spitfire I Rotol) prop had a bit over 23 degrees pitch at low pitch and about 58 degrees at high pitch.
There may be a some minor effect or change due to exhaust thrust or a change in ranking if the positions were close but it is not a big game changer at low altitudes and speeds.