My crack about the Allison was for your benefit (surprise, surprise), actually the Allison birds were somewhat faster below 12,000 ft than the Merlins, but had an operational ceiling of 20k ft. The Merlins could go 30k.

And the business about the limitations of fighter escort is specious as well. Many authors have glamorized the P51 at the expense of other birds, I suspect because it is such a compelling aircraft. In it's appropriate roll it was very capable, but others were good too. Here's an excerpt that explains what the real limitation on fighter escorts was. Politics, not hardware:

During the second half of 1942 and much of 1943, U.S. heavy bombers suffered great losses as they flew unescorted, daylight missions over enemy territory. The new, high-altitude versions of the Mustang have been heralded as the savior of the bombing campaign. But it must be understood that the reason the bombers were not escorted by fighters during the early months of America's involvement in the European theater was a matter of choice and not because of a lack of adequate fighters.

When he was in command of the 8th Air Force, General Ira Eaker did not believe the bombers needed escort. He thought that the bombers' defensive armament would be protection enough. High rates of losses never deterred him from this belief. Accordingly, Eaker turned down the P-38 Lightnings provided him to protect his bombers. The P-38s, with enough range to escort the bombers to any of their targets, spent time in England with relatively little to do, and many were subsequently sent to Africa.