Hi R988,
I think the main reason the Merlin and Allison were/are so good is because of the development time spent on them.
This is true of the 'Hemi' engine, enthusistic development over the last, what, half Century? allows immmense outputs for the design...
I have compared (not in amazing detail) the 2 engines and my findings are on another post somewhere (Turbo vs blower thread IIRC?)
I think the Merlin is better at higher altitudes and the Allison better at lower altitude?
I think this may be likely due to different boost vs comp ratios?
I'm not sure though, I believe radials aren't as good as inlines at high altitudes?
The tank version was the Rover Meteor, yes. It wasn't just a de-blowered Merlin you know! It was made from different materials than the Merlin and was, IIRC, tuned for lower revs/de-tuned for higher revs.
A 2-stage blower engine can use a high-altitude blower and a low-altitude blower. A single-stage engine has to make a choice...
BTW: Me and a friend of mine are working on a revolutionary system, very similar Turbo-compounding...
I don't know about the quietness, "it purred" - Me109 test pilot on the Merlin, but the Merlin was a tough little cookie and I think hard to beat (for a water cooled inline).