OK, the 22mm bar is reported to be more like 20.8mm, if it's the one from BAT, or the BAT unit sold by SP Motorsports. It's still an improvement, but the tech geeks around here say that there's no way to "treat" a piece of steel to make it stiffer. I'm inclined to believe them, no matter what the guy at BAT says.
The 24mm bar is AKA the "Aussie" bar. If I recall correctly (IIRC), it is hollow and lightweight, and is of a stiffness equivalent to a true 22mm bar. It is available from Terry Haines Motorsports (THM), aka, Terry Haines.

Search for him around here and shoot him a PM. You will want (and I believe he provides) upgraded backing plates to use this bar. Some say it doesn't matter which bar you get, the backing plates are a good idea.