Sorry I can't comment on either the iSonic speed W58 nor on iSonic 86 slim, but can do so on iSonic 76 and iSonic speed W53. Both iS 76 and iS speed W53 are 2 different animal, mostly depending on the water state as found on your home spot with high wind.
If your high wind spot is relatively flat water with max 20 cm chop or flat water between 50 cm swell peaks, both are nice to go, althought iS speed W53 is easier to run pedal-to-the-metal with a higher Vmax and better accelerations in gusts (owing to her much smaller water footprint). This is why I finally choose the iS speed W53 instead of iS76 for max speed on ± flat water with both a 6.6 m2 (and either a Deboichet 26 cm speed fin for flat water or the stock Drake 30 cm fin in choppy water) and a 7.7 m2 racing sail (stock Drake 30 cm fin or Select Elite S03 32 cm slalom fin). A pure speed deamon with my light weight in the 12-22 knots wind range.
If your high wind spot is full with steep chop and almost no piece of flat water, a pure dedicated high wind slalom board such as iS76 or iS86 slim is the way to go IMHO. Rounded rails make jibing easier to initiate and to keep track during the entire turn, while the wide tail helps a lot to jibe with higher speed exit. A small Kode might be a good alternative to keep top-notch control, especially if the spot looks like a minefield.
Any speed board (and the iS speed W53 is no exception to that) will become difficult to control in heavy chop. Short and low scooped nose, pretty narrowed tail with boxy rails and small fin makes control a real challenge in well- to over-powered conditions when water conditions worsens a lot. This is why I don't use the iS speed W53 with a sail smaller than 6.6 m2 because chop and swell are getting way too heavy in 6 Bft wind and higher on my home spot. I much liked to take a tiny wave board and a 5.4 m2 wave sail and go ripping the increasingly growing swell.
Finally for your 53 kg, I feel I would rather choose the iS speed W53 because she will still be a floater with her 74 liters for your weight (it's a sinker for my 65 kg) to still be able to uphaul your sail in case of a sudden lull. Since this speed board runs at its best potential only when in well- to over-powered conditions, the extra volume of the iS speed W58 is useless and constitutes just unnecessary ballast for your weight in high wind.