Hi Nick

Does your sail have a lot of luff round? I mean when the sail is flattened, does the mast bend look radical? There were some Cobras in Tassie who experimented with huge luff round on a similar mast a few years ago and they broke several masts.
I think the mast bends so far forward the diamonds can't stop it squirting out to the side.

Or like you said, the mast may have just been very old.

Putting an extra square metre of sail up the top is not insignificant. You may find it a challenge to get enough leech tension without a stiffer mast up top.

Bob Wilson had to run a diamond up the front of his mast, but that is a bit different as he added a metre on to the length.