We do have to be careful about what altitudes we are talking about.
Yes once the British got 100 octane fuel they could get 1310hp out of a Merlin.
2 problems,
1, that was at 9000ft. at 16,250ft it was still a 1030hp engine, even with 100 octane fuel. Official up-rating was March of 1940. Obvouly they had doing testing much earlier.
2. The Climb rating took a while to upgraded. They allowed 2850rpm when climbing for combat conditions in the summer of 1940. RR lists 2600rpm and 6 1/4lbs of boost for 30 minute climb. The 2850rpm limit below 20,000ft and 3000rpm above 20,000ft have the same limitations as the 12lbs of boost. Every use of these climb ratings must be reported upon landing and an entry made in the engine log book.
Take-off power rating was never changed.
Topic is escort fighters. That means they are already done with their taking off and climb to the desired altitude (16-17 kft for the Merlin III powered A/C?). What matters, once they are in enemy airspace, or in a contested airspace, is that they can fight the enemy fighter there. Merlin is a go-to engine for that, as early as 1938.
100 oct fuel helps if the combat descended towards lower altitudes, 10000-15000 ft.
And since these radial engine Hawks showed about 20-22% increase in drag over the P-40 with the Allison what are these 1200hp engines at sea level actually getting you?
Same for the R-2600. 1400 HP @ 2400 rpm at 10800.
Once we figure in the extra drag and the difference in altitude the R-2600 is giving us just about 22% more power than the C-15 long nose Allison (not counting exhaust thrust)
I have mentioned the turbo helping out for a reason.
Experience with A-20 shows that brute force is/was a good replacement for finesse. The 1040 HP C-15 was not a thing in 1939 in-service; from 'Vee's for victory',pg 112:
... the P-40's (sic!) entered service with 950 bhp rating [in July 17, 1940], though this was to change within a year when the Allison 1040 bhp rating was approved.