Yes, it's correct the decvelopment of the iS145 and iS155 are based (origininally) on FType boards. However the differences are also as significant as the "similarities". The difference with the tail (side) cuts is very noticeable in speed comparison (head to head) and the deck shape around the nose/forward deck has been modified to cater more for lighter winds and /or heavier riders.
Then you can take the FT's and race them against the iS's and you will also see evolution at work (or play
Cheers ~ Ian