Hi Nick,
Your approach is interesting, one which I believe you`re not far wrong about (that sails ultimately need replacing more often than most of us do.) My sails are from 2000, but are in good condition because we had them made from the heaviest cloth we could get (not good for weight aloft but good for durability), and sadly because I sail my boat far too infrequently.
I would love to go the squaretop route, but not because my sails need replacing, rather that I believe it will provide a better handling boat with more speed in most conditions.
Your approach that you need compete against faster boats and are not concerned about staying within class rules is also interesting since it allows you to modify things without too much regard for the rules.
With respect to what Wouter suggested, we have considered splicing an additional 300mm of mast section to the bottom of the mast, in effect this lengthens the mast by 300mm, but also moves the hounds, spreaders and diamond wires up by 300mm, assuming you have the hounds and spreaders where they are supposed to be on the existing mozzie mast, I see no reason why the mast will not stand up to the loads, except that there will be 300mm of mast at the base that is not stiffened by diamond wires, perhaps we would need to move the lower diamond wire chainplates down and have longer diamond wires made up, relatively cheap operation (any comments Wouter ?) The reason for this is not to lengthen the mainsail luff, but so that our crew can get under the boom faster while tacking !! These are all just ideas in our heads right now, glad to see an Auzzie trying things out first