It seems the vast bulk of the fleet in particular all the new boats have settled on a 21' mast normally with the 2.8M jib. I think this particular rig was a John Turner development with an a deck stepped M1 mast and adjustable shrouds for raking. Rowsell NSM1 , 2's and others normally had a 21'6" or 21'8" mast. 
But is 21' really the quickest option ? 
I suspect it is a reasonable all round package but would be slower than some other options on different points of sailing and conditions.
Plus with the development of carbon rigs, better raking etc. a taller perhap a max height mast 22'6" might prove quicker on a modern boat ?
My guess is due to the extra 1.5' of mast height it would be quicker to windward and on a very tight reach but would be slower on a broader reach or run due the lost sail area. 
It is possible to calculate force and moments, see link.
Of course it's an expensive gamble, if only I had the money......

Mmmm, the Rules allow a maximum black band to black band measurement of 6180mm, which is 20'3" (ish). Hence depending upon the distance that you have between the sheerline and lower band you could add another 630mm to 730mm, less the small amount for a mast step, etc.

Of course the sail plan Rule tweaks the maximum measured sail area to account for the height of the luff, but if you work through the half heights and then calculate the area, I don't think you lose that much for having the increased luff length.

Perhaps there is no reason why things have stabilised at the current height aside from everyone is scared of upsetting the applecart, or the sailmakers cannot make different dimensions work. Who knows?

Chris Howarth is trying a "tall" rig on "Bronte".  I think its 22'6 with a 3sqm jib.

I know that this is a bit of a broken record, but in Frank Bethwaite's "Higher Performance Sailing", Julian Bethwaite describes an optimum set of rig ratios for an 18 footer. This gives a height of forestay which is just about spot on for a max. height rig with the forestay at the maximum height (4170mm above lower black band). Julian also goes on to suggest a ratio of 67% to 69% for mainsail to total sail area. If this is worked through, it gives you a jib nearer 3.2m^2 ...

Food for thought.

My old Mustard Seed back in the late 70's had a 3.2 jib with a main on a 21ft 6" luff
made it hard for the crew

With a 2:1 sheet and a self-tacker it should be OK ......

Interesting thread...!