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CS props would have allowed for higher rev limits with less strain on the engine than a fixed-pitch unit.
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No they won't allow for higher rpm. It is also not less strain on the engine. CSP's are much harder to aerodynamically balance while a fixed pitch is very easy and will run with less vibration. RPM is a function of the engine speed and reduction gearing. You can change those items to increase rpm to adjust for different propellers. If you mount a fixed pitch propeller that is grossly underpitched for the application, you will also see an rpm increase and very soon have a big bill to pay.
You control the manifold pressure and rpm in a CSP unit. Fixed pitch, you can only control the rpm and manifold pressure is irrelevant.
That is why the RAF amended their definitions in 1937!
With a two position fixed pitch, the Spitfire pilot is controlling rpm and not manifold pressure. The airplane is equipped with a manifold pressure gauge but it serves as a diagonistic tool only. He flys the airplane by rpm setting.
If you read the Spitfire Mk I Operating Notes, the maximum rpm is the same for all propellers.
It seems very likely that RAF pilots were authorized to "pull the tit" on their aircraft using 87 Octane fuel with a lower manifold pressure boost gain. It appears to be independant of 100 Octane fuel use.