I will note that the USAAC wanted only air cooled engines on attack planes starting with the A-12 which was a re-engined A-8 basically. The USAAC did not buy another liquid cooled attack plane until the A-36 and that was a funding trick to keep the P-51 Mustang production line open until more funding could be found for Pursuit aircraft. At the time the A-36 order was placed there was no money left in the budget for Pursuit Aircraft but they hadn't spent all the money in the "Attack" account.
Not actual "proof" but one more circumstance.
Another circumstance, not only did Curtiss put out some ads for Curtiss 75A as a fighter bomber as shown by MIflyer there is a 24 page brochure on the 75A here.
http://www.wwiiaircraftperformance.org/P-36/Curtiss_Hawk_75-A_Detail_Specifications.pdf
See pages 16, 18, 20 & 21
It is undated but judging by the engines used it is probably from 1939. Both P&W and Wright were exporting higher powered engines in 1940.
Curtiss already had the bomb beams, racks and mounts designed up to 850lbs of bombs for the Hawk 75A/P-36 when they were ordering the P-40 (or in process of ordering it) and they never asked for modification or change in the contract to get a better plane for ground attack until well into 1941?
The P-40C got the ventral tank and was in production in March/April of 1941. Due to the internal self sealing tanks in the P-40C compared to the external sealed tanks of the P-40B fuel capacity was cut considerably and the under fuselage 52 gal tank pretty much restored the original range/endurance.
As noted by MiTasol there is a difference between testing something and listing it in manuals and providing spare drop tanks in quantity.
Another circumstance, The P-40 and P-39 were given engines with 8.77 supercharger gears. The engines in the YP-38 and XP-39 used 6.44 supercharger gears. This allowed for about an extra 100hp for take-off and at low altitude. The P-38 kept the 6.44 gears through the E model and then changed to 7.48 gears, the same gears that were used in the A-36 engines.
Allison engineers were not dumb, They gave the Army the best performing engine they had at the time at altitude. They had options available for more power at low altitudes (say 5000ft?), but less power at 14,000ft. The Army didn't want them.
There is some dispute about if the the XP-38 used B-1 or B-2 GE turbos. The YP-38s and through the Es they used B-2s. The XP-39 used a B-5 turbo (?) the P-38F & G used B-13 turbos. They were fooling around with the turbo controllers ( and 3rd supplier) with the Army acting as the coordinator. Turns out the early army designed controllers were crap (fundamentally flawed) and a new type of controller was needed.
The Army was well aware of the state of the turbo engines in 1938/39/40. and even into 1941-42.
The case is very strong that Allison was left behind by developments by Britain and Germany in 1940-42 that significantly improved their aircrafts performance at altitude.
It didn't help that that the US over-loaded the ever loving crap out of the P-40 and P-39 with the P-40E gaining 1145lbs of weight (clean with only 120 US gal of fuel) over the P-40 no letter and using pretty much the same engine (beefed up a little and a new reduction gear). P-40E carried 38% more guns and ammo than a Spit VC or Spit IX.
Without primary documentation for the ground attack or low altitude role I am not buying it.