RE: Board Selection - Totally Confused
First it seems you are "mixing" the preformance characteristics of your heavy Polypropylene Hi Fly Motion, with a centerboard and using a relatively small sail with the performance characteristics of much lighter sandwich construction (or Tufskin thermoformed PVC in the case of the GO Toughskin).
Width does equal stability in both types of boards, but the weight and hull design of the Hi Fly Motion put it more in the same category as the Starboard Start which is signficantly shorter in length.
Your Hi Fly Motion will plane, but due to it's weight, your small sail size, and perhaps some technique issues you aren't quite there yet.
Both the Nova and the GO are as wide (or wider in the =>180 liter sizes) but weigh less and have a more high performance hull and bottom shape.
The GO and the Nova would be the next level for you, but it sounds like you want a board with even more performance and in a wider range of windspeeds.
You could skip to the next level above the GO/Nova and get a large Carve, but for every upside there's a downside and the problem here is even the largest Carve board (Carve 162 162 liters; 253 cm long; 82.5 cm wide 54.1cm 1'off tail) 9.3 kg. Technora Const.; 8.9 Kg. WOOD Const.; 6.1-10.0 m2 sail range) is 7.5 cm narrower and is not going to plane as early as a 90 cm wide GO or Nova 165.
So, you can have a more advanced board that will cover a bit higher wind range with the Carve 162, but it won't be as good in light winds.
So, if you get windspeeds of <14 knots alot of the time, then the wider GO/Nova will be better.
Are you using your centerboard in the HiFly Motion?
The Motion isn't going to plane very well with the centerboard deployed as the CB causes alot of drag and will be unstable at planing speeds.
Your stated windspeeds are 10 k/h (5.3 knots)- 25 k/h (13.49 knot) and in this range you will need a really big sail (8.5 m2 or>) to get any real planing performance on any of the boards you suggest you are looking at. 10-25 k/h is really in the realm of the super wide boards like the Apollo (starts planing at around 6 knots with a 10.5 m2 rig) or the Serenity (doesn't plane at all but goes quite fast very early in like 3-4 knots with a 7.5 m2 rig.).
So unless you can find a windier place to sail, I'm not sure any boards/rigs are going to get you planing reliably with smaller than about 9.0 m2 rigs.
It seems you are stuck at a beginner/early intermediate level by your HiFly and 6.3 m2 rig.
Try a larger (8.5 m2+) rig on your HiFly and it may plane for you when the wind gets up around 11 knots/20 k/h.
What is your weight..... that could be a very important factor as to whether you will plane..... or not.
Hope this helps,