The iSonics were originally specced (and tested) with Race straps in line with the "full race" profile of that board, however it became very clear very quick that the extra control of the Slick strap allowed extra control (=speed) when the iS was sailed really at (over) the limit c/w the Race strap, which (in MOST riders cases) did not allow (due to a relatively "fixed" shaping and minimal fine adjustment) such a locked fit onto the toes at max speed/stutter chop as the SlickStrap.
Foot "stance" on the iSonics (esp sailed at the limit) is subtly different to the stance/style most guys use on FW- where the race strap (and it's fit) are a more acceptable compromise (for many).
Also correct to note that everyone's feet are different sizes/shapes, their sailing styles and conditions are different etc etc, so what works for some may not work for others. In most cases, if a customer prefers an alternate to the supplied straps (in this case SlickStraps), most dealers are happy to arrange a simple change over/exchange at time of purchase.
Send me your SlickStraps, I'll send you some Race
Cheers ~ Ian