RE: What planes faster...?
Roger is certainly the expert in this category, so please listen to him first. But I thought I'd chime in here since I sailed the GO 139 a lot this year. I am fairly light guy at 72 kg, just a little heavier than your wife. I used 8.5/7.3 mostly with the GO 139 and found it to be an early/easy planer. I also used it successfully with sails as small as 6.0/5.4. So not a bad choice.
I have the Carve 122 as well, and find it needs to really be lit up to plane, and it is more of a "traditional" feel to get on plane (versus her GO 170) where you have to let the board get on plane out of the straps (versus being able to have the front foot in on a wide board) and only move back once the board is at speed and the fin has "grab". I would bet (and have read in tests) that the Carve 144 is much like that versus the Carve 121. But at 78 cm wide, I would bet the Carve 144 planes up almost as early as the GO 139, just with a little more technique required.
I would also consider the F-Type 138 for her. I have the 148, and sail it mostly with 9.5/8.5, sometimes 7.3. That thing is an early planing machine! With her small size and likely smaller sails, the 138 might be better for her (88 cm wide versus 96) (for 2007 it is now the iSonic 145). It requires a more technique (a little pumping and awareness to what is going on), but will plane way earlier IMO versus a GO 139 and any Carve. Because of the width it doesn't like heavy chop as much as a narrower board, but my F-Type 148 isn't that much worse in chop that the GO 139. If chop is a problem you typically need to be on something 70 cm or less or even 65 cm or less wide.
At 222 cm short, the F-Type is also more sensitive to front/rear foot placement, but after getting used to it it is no problem.
I think the F-Type 138 would probably match up well with the 6.6 and 7.5 sizes you mention above, and could take an 8.5 for even earlier planing if she wanted to.
I sold my GO 139 and now have the F-Type 148, iSonic 125, and Carve 121, and quite frankly I sail my F-Type 90% of the time as it is in 7.3/8.5/9.5 conditions, flat water to mild chop, and I just love that board.
So the net is, based on what you describe, I would suggest you consider in this order:
1. F-Type 138 (2006) /iSonic 145 (2007)
2. GO 139
3. Carve 144
My other comment is with regards to your comment:
"The second goal is also to get a board that is a bit more responsive to foot steering when she gets planing, but still has good stability."
Stability rank will be F-Type 138 (due to the width, once you get used to the shorter length), then GO 139, then Carve 144, but foot-steering and manuerability/jibing will be the opposite order with the Carve on top, so that complicates matters a bit and maybe pushes you back to the GO 139 and Carve 144 as the top contenders...
The bonus of the F-Type is if you are an more advanced sailor you might steal it from her on lighter days and have a blast on it!
Good luck with your choice...