I'm around the same level as you (maybe a bit further along), but I pretty much only fly foils. I'm sure others will have more specific info for you, but here's my 2 cents...
I have an old Speed 3 21, and have had great sessions in 8 knots on bigger boards. For me 8-13 is the sweet spot. I'm trying to get an extra knot or 2, but at that point it's all skills that I do not yet possess (but working on!) On a crispy new soul with tuned bridles and a door, I would think you could go down a knot or 2, even at your weight. My 21 however, gets to be uncomfortable after 13-14 knots (I'm 85 kilos). I think you could get a couple extra knots on the high end with your weight as well. My 21 is very, very, very slow. It's something you have to get used to. Once it gets moving, it pulls like magic. I would imagine the Soul is faster than my old dog, but still, it's a kite for eking out the lightest winds, and for heavier riders.
I just got a Speed 5 18m, and it's better in every way, fills faster, more responsive etc... low for me right now is around 9 knots, high is around 18. Again, with some skill I should be able to get those numbers down. I'm guessing the speed 5 is much closer to the feeling of a soul, so both the 21 and 18 are probably efficient and handle beautifully.
I'm sure others will echo this, but if your goal is to stay on TT, and want the lightest winds, go fo the 21 at your size. If you plan on foiling it will most likely be too big except in the lightest winds (Regis wind
). A 21/15 combo would be pretty sweet (that's what I have right now in Speed 3s - the big boy combo). Since I'm a bit lighter, I'm updating slowly to an 18/12 or 13 speed 5, Sonic 2 or Soul combo.