The Allison was an inferior engine. The Mustang did not reach her full potential until they were fitted with Merlins
There were those who operated the Mustang in the European theatre, the RAF in particular, who thought the Allison was the superior engine. The RAF originally bought the 'stang as an Army co-operation aircraft, replacing the P-40, hence its use in the low level role. They considered the Allison superior because of the abuse it could withstand and its lower fuel consumption, whereas the Merlin was a more delicate piece of machinery. Such was the respect for the Allison version in the low altitude role that an attempt was made to keep it in production when the Merlin was introduced, to no avail of course. Introduction of the Merlin merely changed an aircraft from one optimised in the low altitude role to one optimised for the high altitude mission.