Even with my 60kg i can save some 0.5 to a knot between 12m and 15m (CA Pulsion); this seems ridiculous but is is not, it is 20 % lower end imrpmovement and can i.crease your ride frequency (it does on my 3 main spots) ; and up to 7 knots i prefer the highest power of the 15m, even if less agile.
For 86kg, no doubt ...
If you keep your 9m, i think that 15m is the est option , provided that you choose a very light kite to extend your wind range. Puslion 27gr or 32 gr (ultimate light wind weapon due to weight and loop boost), hyperlink v2 ultra light, Frs ultralight (but i am affraid of a lack of power, experiemented during my test with std cloth). To me the soul is a good one but not the best for this very light wind (too slow turning and too heavy if you keep your 9m)