RE: What planes faster...?
I would agree, with a 7.5 m2 rig your wife might get a smaller board going at less windspeed due to her light weight now that she has skills.
I can't say whick would be better the GO 139 or the Carve 145, but does she want to sail in more wind, or just be the earliest planing woman on the water.
If she wants early planing, then the GO 139 would be a good choice.
If she wants a bit more higher wind range, then I think a Carve 122 (or perhaps the 133) would be a better choice and not really affect her early planing, on a 7.5 m2 rig as much as you might think.
What windspeed does she get planing, on the 7.5 m2 now?
Hope this helps,