In 2012 I did a side by side with my flex tank with staves in it and a one year old barrel and compared with a carbou of the same wine and aged them for a year.
Yes the free so2 levels dropped at more or less the same as the barrel .
The micro ox effect of the flex tank is noticeable , the wine is smoother and tannins more integrated compared to the carboy.
Compared to the barrel the wine is a bit brighter and more fruit forward.
But in a nice way.
In a blind tasting 4 out of 9 preferred the flex tank .
2 people couldn't tell the difference , me I preferred the barrel but was very impressed with the ft.
This was a Bordeaux blend . But if I was making zin ,Pinot or sirah , I'd probably flex tank it to keep the brighter fruit and light aromatics .
You don't always want any concentration effect , especially if you had any grape raisining / grape dehydration to start with , the preservation of the brighter fruit flavours and lack of concentration effect would be highly desirable in a crazy hot dry year.
For a white wine six months is good , if you need to go longer get the thicker tank. I like the flex tank for six months , it gives the wine a bit more mouthfeel and takes the acid edge off ( doesn't actualy reduce acid but reduces the nails on chalkboard brightness) Great for Chardonnay Viognier or high acid hybrids especially if combining with surlie (natural or biolees) . For Gewurtztraminer, Riesling or Pinot Gris , I'd stick with steel or the thicker tank.