I don't Think extending the mast 300mm will help.
It will make the yacht go faster in light wind, but in heavy it will be way to much.
I say this because When i sailed my northwind against some Storm 100's, For starters the storm
100 is 1m long (Northwind 36 is 90cm), Also the storm 100 has a mast high of 100cm (Northwind 123cm).
Im not sure how much the storm 100 ways compared to the northwind, But i can tell you the storm is a lot heaver.
So anyways, I noticed that when a gust of wind hit my nortwind it would just heal over, until
I let the sails out a bit, Mean while a gust hits the storm 100, doesn't heal over and just powers on past me.
I think this is because of 2 reasons, the sails are to tall, or the lead weight is to light.
I'm not sure what winds you sail your northwinds in, But I would think about changing the shape of the
northwinds Main sail.
The First pic is the shape of sail im thinking of, because the standard sail in my opinion spills wind very low to
the boom. This is ok in heavy winds, but not so good in light.
The second pic is just comparing the to boats on the same tack, same wind gust, just the northwind leaning a hell
of a lot more than the storm 100.
But this is just my opinion, By all means make the mast taller