You don't need proof. Xanax is metabolized via CYP3A4. WGFJ potentiaties CYP3A4. Ergo, it will slow down the metabolism, making it last MUCH longer. WIth Xanax specifically it doesn't make it that much stronger(unless your taking a high dose) however, if you kill the enzymes prior to dosing, you may actully feel the 1mg, not to mention it lenghten the pitiful half-life of xanax(which is why I rarely take them) so that it actually lasts a few hours.
BTW, anything that inhibits CYP3A4 will also potentiate Xanax, which includes Tagament, which is the best all around inhibitor of ALL drugs(Opioids, benzos, and even amphetamines), in my opinion. And on this subject my opinion means a lot.