Not sure where you get your numbers but a quick search of the net yields a kill ratio for the Thunderbolt of between 4.6/1 in aerial combat in the ETO to an overall ratio of about 8/1.

While no where near some of the numbers of other aircraft, it has to be remembered that for the most part the roll of the Jug wasn't air superiority but ground assault and close air support.

Also I'm not sure what kill to loss ratios figure into a discussion about most effective engine design. The Corsair and the Mustang both enjoyed a similar kill/loss ratio of about 19/1 depending on your sources. Does that mean there is no benefit to either design? Additionally even with all of the hundreds of kills each by some of the 109 aces, by wars end the kill/loss ratio for the 109 was actually negative due to loss of all the Luftwaffe's best pilots and the fact that at the end of the war 109 pilots were out numbered 30-50 to 1. So does that mean that inline engines should never have been considered for use in fighter aircraft?

Scott