When I had the ST1300 I fitted a BikeQuip screen and it was fantastic. And you will find my reviewof the screen somewhere on the forum. It's a popular choice as an after market screen for the ST1300. Shillas now has my ST1300 with that screen and I've moved to a BMW R1200RT. The stock screen on the RT was not too bad, but for a long haul tourer it left a lot to be desired, so I settled on the Puig Screen for the Beemer after seeing the wind tunnel test results on thier web site. The Puig on the Beemer is a great screen and give good protection and is very quiet, which is what I was after. With both the BikeQuip on the ST, and the Puig on the Beemer, I am still looking over the top of the screen, and the little bit of distortion caused by the recurve at the top is not noticable.
I'd go for the BikeQuip as its improved performance is proven on the ST.