RE: Isonic - Width
iS155 has a narrower tail (67cm) than FT158 (73cm) and iS is not really configured to carry FW 70~size fins in the way the FT158 could.
but at 80kg if you've got a real 10-15 kts, you should have no trouble with the iS155 for planing in that range with 10.3m. The iS155 gives you a better jibe, a design more suited to faster reaching speeds (in tlighter wind range) and low drag tuning at the expense of the ability to handle the super big fins and pull the extreme upwind angles of the FT158. However the harder (and more efficent) rails of the iS (c/w soft/er rails on FT) do offer some extra edge for the rider who can make advantage from that, so the on water upwind difference is not as much as on paper.
Cheers ~ Ian