Just to clarify, water is probably one of the most common solvents that the majority of people use on a daily basis (sometimes referred to as the "universal solvent"). A solvent is simply something that soluble solutes are dissolved into. Water is unique in regards to kava in a number of ways, notably that it is the only solvent to extract
any glutathione, at least compared to acetone, ethanol, hexane, chloroform, and methanol. In addition, it also extracts no Methysticin and somewhat lower amounts of flavokavains, although not nearly as dramatically as Methysiticin and Glutathione.
Furthermore, one of the primary aspects of what makes acetone and the like undesirable is the fact that they are very nonpolar and therefore extract nonpolar substances readily compared to, say water. CO2 is also a nonpolar substance, but due to supercritical extraction methods, which I'm not familiar with, their could be a lot of variance from organic nonpolar solvents like acetone. The way I understand supercritical CO2 extraction, CO2 is acting as the solvent...
I have access to lots of data regarding the efficacy of extracting over 7 dozen various compounds in kava rhizome, but under copyright laws I can't publish the tables in a public environment. That being said, I can give some more information regarding the efficacy of various solvents at extracting kavalactones, flavokvains, and glutathione.
The total phenolics varied considerably among the solvents tested. Chloroform showed the maximum phenolic content (63.1 mg GAE/g extract), followed by the acetone extract (43.0 mg), water, methanol/ethanol and lastly, hexane. The amount of total phenolics in the water extract was markedly reduced relative with that of the acetone extract. However,Interestingly, methanol, ethanol and hexane extracts yielded a minimum of total phenolics (2.9, 2.9, and 2.2 mg, respectively). In summary, the regarding maximum yield by a given solvent, the list in order of decreasing efficacy was chloroform
< acetone
< water
< methanol/ethanol
<hexane extracts.
That being said, water extracts 0.0 mg/g of Methysticin (found in aerial portions of plant) whereas every other solvent tested (other than ethanol and methanol) extracts significant quantities of Methysticin. There are also interesting tidbits regarding specific kavalactones' ability to be extracted in different solvents.
Then of course, there are the flavokavains... but I hear that's a dead horse so I won't bring those up.
[
http://www.ncbi.nlm.nih.gov/pubmed/18404321]
Xuan, Tran Dang, Masakazu Fukuta, Ao Chang Wei, Abdelnaser Abdelghany Elzaawely, Tran Dang Khanh, and Shinkichi Tawata.
"Efficacy of Extracting Solvents to Chemical Components of Kava (Piper Methysticum) Roots."
Journal of Natural Medicines 62.2 (2008): 188-94. Print.