Maximum height of mainsail
Can I check my sail plan rule understanding is correct... If using the more usual 5720mm luff main - the rule that the lower band must not be more than 730mm above sheer, the maximum hight for the lower edge of the top band (mainsail) is 6450mm (5720 + 730) above sheer-line?
All sounds entirely correct. The formula relating luff length to sail area means that a higher rig and lower sail area will give slightly better windward performance but will be more difficult to hold up in a blow and a bit slower downwind. The compromise has ended up where it is by a sort of natural selection mainly based on winning the champs on the sea. As playing with rig dimensions means a new mast & sails experimentation is not cheap and the gains uncertain there has been little recent change, though as you say the river sailors go for tall rigs.
JT has a taller rig I believe, not 22'6" but somewhere between 21' and max. There has been very little experimentation with taller rigs on new boats.
Many talk about this as if its one or the other - i.e. 21ft or 22ft 6" but it seems there is no reason not to go in between? I guess it makes sense to have the mast at least capable of the 6450 (21ft 2") max or perhaps a little more height and then put the top band in the most appropriate place in line with the rule.
Lots of 80's NSM2's etc had a 21'8" set up