If the time line for US-built Merlins can be shifted 3 months earlier than a historical time line, that would mean P-51B (or what-ever would be called) would enter the fray in Sept 1943, rather than Dec 1943. Of course, if the USAAC bras introduces them into as an offensive tool together with B-17s that early. By that time the P-47 is also fitted with drop tanks.
Prospects for the LW don't seem bright in this scenario.