The during the Battle of Britain the Germans noticed improved performance of British fighters and became convinced that the Brits had "up engined" their Spitfires and Hurricanes. A Spitfire was eventually forced down nearly intact in France.
The Germans went through it finding nothing changed until they analyzed the fuel. Addition of tetra-ethyl lead to boost the octane of gasoline had been discovered in the US in the late 1930s and had been held as a military secret.

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Most of the upgrades to the Merlin were the result of ever-increasing octane ratings in the aviation fuel available from the US, and ever more efficient supercharger designs. At the start of the war the engine ran on the then standard 87 octane aviation spirit and could supply just over 1,000 hp (750 kW) from its 27 L displacement compared to 1,100 hp (820 kW) from the 34 L Daimler-Benz DB 601. From June 1940 small quantities of 100 octane fuel became available from the U.S. and Merlin IIIs were found to be capable of running on it.[2]

The next major version was the XX which ran on 100 octane fuel. This allowed higher manifold pressures, which were achieved by increasing the boost from the centrifugal type supercharger. The Merlin XX also incorporated the first of the two-stage superchargers designed by Rolls-Royce. The result was 1,300 hp (970 kW) at higher altitudes than previous versions. Another improvement made to the XX and future Merlin variants was a redesign of the cooling system to use a 70/30% water/glycol mix rather than the 100% glycol of the earlier versions. This allowed them to run some 70 degrees C cooler, substantially improving engine life and reliability. It also removed a fire hazard from Merlin powered aircraft, as pure ethylene glycol is a flammable liquid.[5]

The process of improvement continued, with later versions running on further-increased octane ratings, delivering higher and higher power ratings. Fundamental design changes were also made to all key components, again increasing the engine's life and reliability. By the end of the war the "little" engine was delivering over 1,600 hp (1,200 kW) in common versions, and as much as 2,070 hp (1,544 kW) in the Merlin 130/131 versions used on the de Havilland Hornet. By mid-1944, the Merlins of most fighters flown by the ADGB and 2 TAF as well as those of the USAAF were running on "100/150 grade" fuel. This new rating was achieved by adding 2.5% Mono methyl aniline, or M.M.A, to 100 octane fuel; tests had determined that large quantities of tetraethyl lead resulted in excessive fouling of the spark plugs through lead deposits.[4] [6]The new fuel could be recognised by its bright green colour and the "awful smell".[7]
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I had a chance to talk to a batch of old oil wildcatters up in Alberta once... the spoke of all the Merlin engines that were available after WW 2, when all the British bombers were sold off as 'scrap'.. Alberta had been loaded with RAF flight crew training bases.. and a lot of Lancasters ended up being scrapped there..

the guys who pumped oil out of the ground used the Merlins to do so...

they really spoke of how much spunk and power Merlin's had...with time, the used one's price came at a premium for the amount of work it would do, and its durability...even when they blew a hole in the side of the engine block, during the middle of a pumping session.. not to loose compression in the lines, they would just keep pouring oil into the engine as it was squirting it out the side of the block... but it would keep running...

diesel engines finally replaced them when they were all but worn out across Alberta, but the Merlins supposedly could pump up a well in about 25 to 30% of the time it took a diesel to do so..

Those Merlins were definitely some amazing well built engines..

Looks like R&R and the British are continuing this tradition.

We are now operating two squadrons of Apache Longbow, known as the Apache AH Mk1 to us Brits.

Impressed by the AH64D, the boffins at Westland decided they could improve it further. Out went the old power plant and it went a pair of Rolls Royce RTM 322 which delivered a staggering 25%-30% more power.

The biggest limitation is using all that extra power is the existing transmission system and it will take a major redesign (under way) to enable the aircrafts true potential to be utilised. Presently the extra power of the AAC Apache is only used for short periods but that has been enough to greatly impress the pilots...

Again in a case of history repeating it self, I believe the USAF is looking at introducing the RR engines into their Apache AH64D's with Boeing leading the R&D of the new transmission system..

It is an endless cycle of hot rodding.
Once they get to testing them they will find they no longer have the range due to greater fuel consumption then they will have to increase the fuel capacity increasing the weight slowing it down some. Then other things like the tail boom maybe found wanting and you eventually wind up with a lard ass helicopter that takes more fuel to do the same thing.
It is always a compromise between speed, agility, endurance and reliability. And somewhere in there cost starts to be a factor.