At this point I just have to chip in... Weight is significant, but shape, rails and rockerline is much more important! Early planing comes from a straight rocker, a hard and sporty feeling on the vater often comes from minimal vee. On the other hand, a board with deep double concaves often feels very soft on the water.
With regards to your SUP, the reason it does not release properly is not because it is heavy, but because it has a lot of rocker. A Kona One is equally heavy , as is a Phantom raceboard, but both of these boards are fast thanks to a relatively straight rocker.
The Kona One is 350 cm long, has a weight of 16 kg or so (probably more), yet I am faster on it than most freeride guys (and much faster than those on Freestyle boards!).
Claim: construction makes a difference, as does weight, but both are only secondary factors.
I think that in order to do the really lightwind play you should get yourself a Severne Glide 8.5 (I think it will rig well on the mast for your Simmer), and a iSonic 141 (or something in the same size range) or a Futura 144 (may be more forgiving to lesser sail stability than the iSonic). The Glide is made exclusively to push down the planing threshold, and should equal a 10 m2 regular sail. I think that a board smaller than iS 150 will be more fun immediately when teh wind has filled in a little.
And then I do think you should switch to a 105 liter FSW for your "over 20 knots" sailing. It is a waterstart board for you, but floats you comfortably even with very little sail pull. And is so much fun!