Hi Giles, I think you should check availability on the 3.7 SLS, I could not get one as sold out/am sitting quite close to the source...:-)
That range sounds good - personally I always prefer 3.7/4.2/4.7/5.0/5.3, mostly I find the sails always seem to work best with a bit of extension (I ride the 3.7 with the 340 mast? but your 3.7 ,4.0,4.5,5.0, &5.3 will also work for sure. Not sure if you really need a 5.7, as the Grip 3 has so much planing power/that sail size tends to push the nose down.
If I had to choose then I'd drop the 5.7 and slot in the 4.0, as the gap between 3.7 and 4.2 is quite big in %/that's a size which could be breaking in some conditions, so it's good to have a backup? 3.7/4.0/4.2/4.7/5.0/5.3 is a great range...I hardly ever use my 4.5, but we are 4 people windsurfing in the family, so I need all sizes..:-)