The iS87 would be a good slalom/speed option for 5.8/6.4m vMax chases, and yes it is capable to vMax of 40+ in speed conditions.
You'd probably do it easier on iS50 in solid 5.8 conditions though, although agree 50 is not as versatile for 6.4 or general hiwind blasting, and certainly more problematic in marginal conditions ;iS50 wouldn't be a consideration to obsolete the ST104's range.
iS87 + 115 would be good pairing range wise, and suitable to obsolete the ST104 if you were concentrating on speed/slalom more than fast freeride / freerace chop type riding; ST104 adds that comfort/control edge if you were out blasting in 6.4/5.8 chop - and looking for a more relaxed ride c/w the committed speed ride from iS87. However it seems you are focusing on flat water for slalom / speed and then armed with K96/FSW84 for rough, in which case the ST104 might not be often out in it's best niche, in which case it could be a rollover victim in favor of that 87..
2 Fins vs 1 ?? Bam ! That's a good question ! As with the HyperSonics, the larger iS's really do have a wide/r use/tuning range than the more dedicated iS87 and 94, and so the justification is stronger to package the iS101 and larger with two fin options. "Ahhh", I hear you say, "so then what about that iS50?" Well there's a bonus one; Slalom Pro 280 adds light (speed) wind performance and doubles as "slalom" fin when the 50 is used by kids (big or small) in more "normal" conditions ; Speed 230 is more specialised for the real deal - serious downwind vMax.
Cheers ~ Ian