RE: low planing question :)
I think if you can get going on the FT-158, with a 10.0 m2 rig at 85 Kg. (187.4 lbs.) your wife ( 59 Kg./ 130 lbs.) would get going on her FT-138
with an 8.0-8.5 m2 rig. Be sure you get her a light weight free race type rig with good low end power characteristics.
I cannot answer your question about the Isonic 155 (or any of the '07 boards) as I have not sailed any of them myself yet.
I will ask your patience here for about another month. Then I will have some of the new '07 boards and can give you my impressions of them.
I very much try not to suggest anything, until I've sailed it myself to figure out if the "marketing" is on target or not.
Hope this helps,
P.S. You are most welcome for any and all help you've received over the years. It's always a pleasure to make suggestions and then get feedback that at least some of them were on topic and on target and they actually were somehow beneficial.