Perhaps you're right. Even if we got a combat capable version in 1941, the 1200 HP at 25000 ft acomplished by turboed radial are probably as good as 1100 HP at same altitude acomplished by turboed V-12 engine (less drag), and 1000 HP by non-turboed V-12 (less weight, bulk and drag, use of exhaust thrust). On the other hand, the American's don't have a V-12 in 1939-41 that can do 1000 HP at 25000 ft, so it is a turboed either R-1830 (that was done, the result was less than spectacular) or V-1710 (went good on P-38, not that good on XP-39, problematic in XP-37, probably usable but too late in XP-60A, unfortunately never materialized in XP-40H).
USAF can also use the R-2800 in non-turbo installation, either as single- or 2-stage versions, that would give them a very useful performing 1-engined fighter. Sorta the early Bearcat with a thinner wing and no fancy blow-off wing panel, or Tempest II-looking fighter.