My wife is 5’7” (measured now) and tops out at 135lbs. My friends at Blizzard agree that the 172 cm Sheeva is too much of a ski for her. That ski is designed for a strong and highly skilled female skier. Many people say, ‘go big’ without considering their height, weight, and skill level. Trish suggests that the Fisher RC ONE 82 at 166 cm is an appropriate length. The shorter 158cm seems a bit short for her.
At her height, weight, skill, she’s going to probably fall in between a lot of size runs...speaking from experience as a similarly sized human (I'm slightly shorter at 5'5", but same weight). There will often be times where she needs to decide between going a bit too long, a bit too short, or just choosing a different ski. If the opportunity comes up to demo...I would recommend trying something longer in the freeride category. She might not know what she's missing. With that being said...as AmyPJ put it, if she likes them in the sizes she has them in, I also wouldn't sweat it or actively seek out the opportunity.
My original thought in asking you what sizes she is currently skiing is because I normally size a front-side ski 5-10 cm shorter than a freeride ski like her Ripstick/Sheeva. I didn’t want to recommend anything that would be too long based on what she’s used to.
For a status point from someone close to your wife's size, 5 years ago I would have considered myself to be a low advanced skier...the ski that really upped my game was the RTM 84 in a 162. I still ski it. It's stiff enough that I can go that small in size, and still have enough stability. I have always (since I got back into skiing 12-ish years ago) found upper 160s and low 170s to be my happiest freeride length. My powder skis are 176, but fully rockered.
On the RC One, I would probably lean towards the 166 as well, but I think that’s a big step up in length. The Wildcat will be softer than the RC One and thus, I would think the 166 should work for her well. The Head V8...I think she is very much in between the sizes; I would probably cross it off the list for that reason. If you want to go with a Head ski, I would probably look towards a stiffer version and go shorter (maybe a Supershape in a 163).