Point of fact, the P-51 Mustang did not use actual Rolls Royce Merlins, because Rolls Royce could not keep up with US production demands, and traveling across the Atlantic was sketchy at best due to the submarine wolf packs. Instead, Rolls Royce licensed the Merlin 66 design to Packard, which was built under the designation V-1650-7. 

The P-51B was the first to use the Packard engine.  This engine made about 1,300HP.  Packard made some changes to the engine that both improved ease of production and durability.  They created special main bearings that used indium plating, developed ironically by Pontiac, which prevented corrosion; a common point of failure due to the oils of the time. 

The P-51 was built in four variants A, B, C, and D.  The D variant gets all the glory because it was the last and most successful.  The airframe design was one of the first to use the laminar flow wing, which was incredibly efficient, and made the P-51 so responsive and maneuverable.  The major change to the D variant was the introduction of the bubble canopy, which greatly aided pilot visibility and improved dogfighting. 

There were some alleged performance issues with the A variant due to their use of the Allison V-1710 engines which were only fitted with a single stage supercharger.  This limited high altitude performance where the air was thin.  The Allison gets a bad rep in my opinion, because the Army never asked Allison to improve high altitude performance (which was easily corrected), they just replaced it.  The design was inherently good, durable, and easier to service than the Merlin pattern engines.  It was also 355 lbs lighter than the Packard V-1650-7 that replaced it.  After the war the Allison lived for decades in unlimited hydroplane racing, tractor pulling, and air racing where it broke many, many records.  The Merlin was used in these sports too, but its cost and complexity made it less attractive when the V-1710s were lighter and made more power closer to sea level. 

Many years ago my uncle wanted to fit a V-1710 to a wooden speedboat.  The project never reached completion, but I did get to work on that V-1710 a bit, which only furthered my interest in WWII era aircraft.  They were the space shuttles of their day, absolute bleeding edge technology.  Awesome stuff, just awesome!