Merlins never had roller rockers,a problem with cam and follower wear,a set of custon roller rockers can cost 30,000 bucks,ouch.
Many piston engines had compound supercharging to deal with high altitudes.In reality,high flying brought on lots of problems with ignition and detonation.
Generally,in a compound supercharger setup,a mechanical or turbo blower supplied air at sea level pressure to the secondary blower reguardless of altitude.the secondary blower then boosted that air to whatever pressure was necessary for the power level required.Intake pressures of 100 HG were common late in WW2.The blowers consumed upwards od 300 HP from the engine(s).High altitude rated Merlins had around 1200 HP available at the prop at 30,000 feet.
Room in aircraft was tight,the size of after coolers to keep incoming air at reasonable temperatures was limited,making water- alcohol injection a requirement on some.Boost controls were crude,reguired judgement on the pilot in many cases.Piston failure by detonation was not uncommon, not the best thing when you're flying over hostile territory.
