Also, remember that the Grumman XF6F-1 first flew on 26 June 1942. Even at that time, it was already obvious that the 1600 hp R-2600, in either its R-2600-10 version in the XF6F-1 or its R-2600-16 version in the XF6F-2, was not going to provide the speed desired for the airplane. The solution, which Grumman had already identified, was a switch to the 2000 hp Pratt Whitney R-2800 engine. The XF6F-1 flight with the R-2600-10 was essentially a test to verify what Grumman and the Navy already suspected - the need for more power. The XF6F-3, the re-designated XF6F-1 now mounting the R-2800 engine, first flew on 30 July 1942.
Up to that point the USN had only observation estimates of the A6M2's speed performance. Twenty days later, the Koga Zero arrived in San Diego, and after about a month's restoration, A6M2 #4593 finally got in air over North Island. The results of the tests conducted between 20 September and 15 October by gave the Navy a definitive picture of the airplane's capabilities and weaknesses. Speed wise, taken directly from the Sanders initial evaluation report of September 29, 1942:
Maximum speed.. Sea level.........270 (m.p.h.)
.........."..........."........5,000 feet.…..287......"
.........."...........".......10,000..."........305......"
.........."...........".....*16,000..."........326......"
.........."...........".......20,000..."........321.5..."
.........."...........".......25,000..."........315......"
.........."...........".......30,000..."........306......"
As you can see, and as Twitch noted, the XF6F-1 with the R2600 was faster a sea level (315 v 270) and generally faster than the A6Ms best reported speeds. What the Navy wanted was 'more speed" and the R2800 gave that to the F6F-3. The Navy, with remarkable prescience, placed its first order for the F6F-3, with the R2800, on 23 May 1942, before any version ever got in the air (and a scant two weeks after USN fighter pilots had encountered the A6M2 for the first time at the Battle of the Coral Sea).
R