For Ken : Are you only experiencing the spinout mid jibe ? Obviously with a wider tailed board (like iS) there is more potential to spinout or slide the (wider) tail mid jibe at load/speed than say a similar sized freeride with narrower tail and/or even less fin. Generally the 2008 stock fins are quite OK, we use them a lot in testing and if they have too many vices it's pretty obvious
Technique might also be a contributing factor, for the best results you really need to set up well on entry with back hand (and then rig) raked back but NOT opening the rig on entry and then keep a lot of drive(preessure) on thru the mast foot mid jibe. Even on the (wider) 122 it's possible to pull some rather insane jibes (for 120Lt pure slalom ) without having to "overfin" (for the conditions) the board to achieve it.
* Yes, the above recommendations are downwind slalom specific - which tends to run higher speed and less fin than typical, Fig 8 or upwind. In super light wind (and to some extent very high wind) you won't reduce the fin size too much for iS but in the critical mid range you would not want to be on too much fin for downwind.
For Rafal : Yes, for sure, the SL3 is definitely an option for increased control in the higher winds / choppy water (and so the small fin/s of your quiver). For SL3 you can use basically the same size as SL2 (SL3 is basically as SL2 with raked tip section and slightly more area but slightly less efficency ). SL4 is a promising new design with less lift (but more control in flighty conditions) than the SL2 or SL3, in general for iS you would use slightly more fin with SL4 than SL2/3. Most riders find the iSonics tune up best in pure performance with slightly more fin power than "average" - that's to say taking advantage of a little extra power/lift, but of course this makes for a more highly loaded ride; fast but maybe not so easy on the ride in rough/choppy conditions.
For Expander : for the older (slightly narrower) 2007 iS111/133 you would use similar fins but with a slight emphasis on the -1 to -2cm end of the tolerance. That's to say, the same or a little less will be ideal ; 2008 iS in these sizes can take just a little "more" fin for a given range than 2007 and still be OK/ideal. It's not normally a full size up, to keep it in perspective though (and the difference is les than say those caused by rider weight/style , water conditions and course upwind/downwind etc) .
Cheers ~ Ian