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I have gone down a rabbit hole on this. Lee Mcormack is one of the few people to have considered dynamic bike fit. Most people say “just follow the manufacturers size charts” despite huge changes in stack and reach over the last few years.
Under the ridelogic fit system the most important dimension is the rider area distance then the angle (for type of riding) and then the steerer offset for ease of turning.
About 5 mins into this video they discuss rider area distance, get it right and pumping etc will become easier.effectively locked out straight arms and legs will bring the bars up to your hips giving maximum power to that lift of the front end.
After that comes the angle from bb to bars - lower angle (56 degrees) better for climbing and sprinting (eg xc), higher angle (60 degrees) for descending (downhill). So general trail riding is somewhere in the middle.
Steerer offset if reduced to a low number makes turning the bars easier as you are not having to reach so far forward (if it’s out front like an old style xc bike)
So it’s not that I want a super short bike but I am on the cusp of sizes between birds medium/large and large sizes. If I raise the bars on my large it will compromise the most important measurement on the bike so that does not work.
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dancing james
Could you test ride a smaller one, back to back with your current one, before making a financial commitment to change?
I accept that e.g. Jack Moir, Bernard Kerr etc are on smaller bikes than is now conventional for racing - but they choose them because they're less stable, IIRC Moir rides a bigger frame when he's not racing.
I'm put in mind (hear me out on this, I admit it's a stretch) of modern jet design being to deliberately make something that is dynamically unstable which is then made flyable via the electronics, the results of this combination being very high manoeuvering potential when compared to an inherently stable design.