I don't know the Tushingham Lightning at all. But if I guess the sail has too much lift for you when the wind increases. Mastbase, stance, fin is rather hard to say what is right or wrong on a forum, this could be already quite good (I suppose your trim is good below 20kn??).
Therefor I suggest the easiest solution (with high% of succes) would be adjustable harnasslines, just make the lines longer if the nose is too high when the wind increases. Lowering boom can help, but also could have the opposite effect (nose even higher), this mainly depends how much you drive your gear through your harnass. Check out how it works for you.
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2011 Starboard iSonic woodcarbon 107, iSonic speed special W54, iSonic speed special W44
Severne Reflex Sails