RE: iSonic 94
As an allround slalom machine, the iS94 has more versatility than the (more higher slalom wind focussed) iS87 - but as a medium wind speed board the iS87 has more potential than the iS94.
In a lot of cases the crossover from iS94 to Missile sized boards is not so impractical, especially if the ride style for the iS94 is more allround slalom (ie; tends to be a bit bigger board for given conditions than pure dedicated speed style riding) - while dedicated speed boards tend to size a bit smaller than allround slalom boards would (for the same wind /sailor range). So the visible "Lt's" gap on paper is not so bad in real life, at least in these conditions.
However, if you're not pushing for major bottom end range on your medium wind speed board / all round slalom machine - and you are looking for some more top end potential - especially before transition to Missile, then there's no doubt the iS87 will definitely deliver a bit more speed board advantage than the larger iS94.
That may not have delivered a definitive answer, but hopefully it allows you more info to make one.
Cheers ~ Ian