At the outset, although I think you will find some overly strong opinions on this forum as well as other forums, it is not quite as easy as saying a 22mm hollow sway bar is equivalent to a 19 mm solid sway bar without knowing more about the bars in question. For example, you would need the nominal wall thickness as well as the arm lengths (for lack of a better term) to actually compute usably comparisons between bars.
Anyways, I digressed. I have had the OEM 16 mm bar, the STI 19 mm bar, and the 22 mm Megan Racing Bar on my 5th Gen LGT. The difference between the 16 mm and the 19 mm was less noticeable than the difference between the 19 mm bar and the 22 mm bar. The 22 mm bar has been great, and has reduced roll without generating problematic oversteer. Frankly, I'm really happy with its performance (and, I was really unhappy with the suspension of this car a couple years ago even with the Bilstein/RCE Black suspension http://legacygt.com/forums/showthrea....html?t=239145
FWIW, I went with the 22 mm Megan Bar because (1) it was significantly cheaper than the Whiteline, and (2) the rear Hotchkis bar I had on my 4th Gen LGT was hollow, 22mm, and awesome (part of msprank's group buy).