2750 vs 3000 ft is a ~10% difference in height, but it results in a less than 1% difference in density ratio.
Interesting that Lumsden states that the 45M has a maximum power of 1,580 hp at 2,600 ft on p210 cf 1,585 hp @ 3000 ft in Part 4.
I noticed that the RR book has the wrong impeller diameter for the Merlin 55MA on p.157 but correct on p. 148.