A softer top should give more control, a stiffer top more low end power.
Tushingham has a flextop bend curve, while Severne has a stifftop. Not a match made in heaven. There are quite a number of other sail brands that would fit your mast better!
This table may be useful for you: http://www.unifiber.net/2011/news/ma...ersion-2011-1/
But I don't have any first hand experience!