I suspect there are a multiude of minor contributions. However, - I`ll allow everyone here to add their own significance to it - but Supermarine WERE in progress to adapt the Spitfire airframe to the two-speed Merlin XX (as discussed, its a very obvious thing to want to do), however they were instructed part way through this work to abandon it - because the Typhoon was "due" to reach service before the convertion+retooling+production would have been completed.
In the end due to the Sabre production problems (and some airframe issues too) the Typhoon was seriously delayed, so that put the Air Ministry in an "Oh S**T" position, where they had no Typhoon, and an even MORE delayed 2-speed SC Spitfire because they had somewhat over enthusiastically told Supermarine to not bother (there was alot of high-optimism in the Air Ministry about the hoped for performance from the Typhoon at that stage - which didnt do as well as hoped at medium-high levels, so they resorted to trying to turbocharge it).
So the Merlin 45, was in many respects (but not all) an "oh S***T" response to having made a bad choice in telling Supermarine to stop its convertion to Merlin XX.
I cant lay my hands right at the keyboard this moment on all the documents for this (which I do have) but here is one which tends to corroborate the non-fitment of the XX into the standard spitfire as being a problem.
If someone really wants it I`ll try to dig out the memo on Supermarine XX work cancellation and the Typhoon being late.
All the best