RE: Sonic 85 v SType 93 v ISONIC 87
ST93 is definitely a fun, fast thing and does have a very strong universal "mid" conditions range - a bit like the iS in range terms, but definitely an easier thing to play around fast on, not that the iS is a handful, but in rougher stuff the ST is more relaxed.
ST93 definitely rides "bigger" than the iS87 or 2006 S85, despite both those Sonics having extra efficency which tends to allow the rider to run a "smaller" pure slalom board in similar conditions to the ST93.
Speed wise, the ST93 is pretty quick against the S85 (or iS87) in rough and confused (dirty) medium conditions,and can be fired at relative random without too much care; ST pushing the limits based on a healthy dose of control mixed with speed, but as the conditions allow for higher speeds, the iS87/S85 ultimately will top out faster in most powered situations. Obviously the usual provisos apply ; ST having the bevelled rail and fuller volume tends to ride higher, less locked, less technical, easier speed than the S85/iS87 - and still allows a reasonable chance at some modest playful moves along the way. Also a stronger layup on the ST (c/w S or iS).
Early planing is quite good on ST93, especially considering the bevel rail which normally detracts a bit in this area; no, the ST does not have the sharp edge of the s85/iS and some riders notice that more than others, inc when pushing either board onto the plane.
The tricky bit (in your situation) is that in many ways the effective sweetspot range of the ST93 overlays heavily that of the iS101
(depending a bit on how you sail/tune each board)- and the ridestyle character of both iS101 and ST93 (although technically different) are basically pretty similar - blasting type boards if you're not racing them.
(the overlap will be less if you tend to use iS101 for bigger sails like 7.6m+, but the overlap will be a lot more if you are mostly on iS101 for 6-7m racesails).
ST93 won't really replace K86 in the rougher or windier (K86) stuff in B&J terms, but you may not be getting a lot of that from your comments (??) and if you're keeping the Evo75 then you're not exactly stranded in that situation either. If you're happy to jump onto the Evo relatively "early" in the waveboard range, then the lack of K86 won't be too bad in the quiver either.
Not sure if that leaves you happy or not(??).
Let's know if you would like further info.
Cheers ~ Ian
PS : Fins for the ST93 : you don't need a lot; best seem to be fast freerides, a bit more rake than slalom compliments ST nicely and doesn't hurt speed too much. You will find the ST93 likes a bit more fin than the S85 ;In the 07 ST93, the Swift340 is pretty good, covering the mid-upper ST93 range, you might also like to add a lower AR 27-29cm freeride for playing around in more powered conditions.Smaller than that you probably will be on the Evo.