We need Calum Douglas, Secret Horsepower Race. He and others may offer thoughts on metallurgy, valves poppet and sleeve, lubricants and superchargers. May I offer some points on the business side, where the established "Ring" of qualified designers simply prejudiced Defence of the Realm. RAE/A.M funded Hyper Power amply; industry stumbled amply. “Peregrine, Vulture, Sabre, Hercules VI, Centaurus, Griffon are outstanding examples of (hopes) disappointed or deferred” Postan,Official History, War Prodn,P167. Napier Design Dept. “devoid of drive...inadequacy (MAP) forced through a drastic reform.” Postan, A/c Prodn. Quality, Pp.37/8/133.
Ministers, seized by the notion that "draughtsmen" Ruled OK!, were in short supply, declined licence Proposals from wannabee newbies (for small engines and for Hispano-Suiza, P&W); Fairey spent £100K, with a modest A.M. contribution to run P.24 Monarch (2,200hp, 1938/9). But the workload of bringing <2,000hp engines to a level of reliability and ease of fabrication fit to meet Rearmament demand meant that the Ring could not also do Hyper. How did Hives think RR (ready to exit Aero in 1935) in 1939 could turn greenfields into high-volume producers, while trying to develop multiple new types, whilst ever-improving Merlin? If he banished his best and finest Managers thither, who is left to mind the home shop?
What Ministers did was:
* to give Napier a new Agency Factory in Liverpool, staffed by bright girls wishing to be taught how to give our boys the tools to do the job - Sabre engines. When Napier failed to do that MAP put in a Management team from Standard Motors. When that didn't work MAP invited English Electric to buy Napier;
* urged HS Group to get a grip on ASM to produce and support Cheetah/Tiger and to stop wasting effort on various Hounds (upto 4,000hp) when they couldn't churn out puppies;
* culled RR's aviary, to attend to more Merlins, to make Griffon work, asked them to thank MAP nicely for giving them Crewe and Hillingdon Agency Factories, and now please to aid Ford and Packard (RR had tried in 1939 to drag their feet on aid to imposed second sources, asserting proprietary interest in the product). There was a War in prospect.
* explored licenced Centaurus in GM and Canada, Bristol seen to be slow and already attempting yet bigger Orion; and:
* to accelerate US production, truly vast sums were spent (by France, too) on expanding US capacity (a "British Wing" was built at P&W/Hartford).
So, to OP's Q: what was not done, that could have been done? Well: choose in 1937-ish one of Bristol, RR to do Hyper, the other to stay with Hercules/Merlin.
It was truly shocking that MAP built at humungous cost an underground factory at Corsham to build Centaurus even if Patchway were bombed, yet so slow and bugged was Centaurus that its sole combat success was U-927, 24/2/45, Warwick G.R.V. But if that had been foreseen and all bets placed on Griffon...would that have arrived quicker?
I think the A is...nothing really.