22mm is the way to go. I changed mine to a hollow 22mm Ireland Eng. and WOW... what a difference - a lot more neutral, settled and predictable car. Lots of fun!

 

I have the 22mm with HSport, FSD and M7 strut tower. The Alta 22mm is set in the middle and works perfect with my tire set 225/35/18. I was racing through some switch backs the other day and I was able to get my rear to drift a bit. The car is very netural and everyone thinks the car is All Wheel Drive because it doesn't drive like the typical FWD car. Also I think my Quaife LSD also helps the front grip and pull through the corners.
Not sure if this helps but it works for me, I guess whats perfect for me is wrong for someone else.
M

 

Alta 19 mm on stock S susp and 16" runflats.

very nice improvement, tighter, but not unsettling. Started on full soft, came home after a day and set to full hard. For me, it's just right. Spend a lot of time driving a turbo miata with an oversized rear bar that liked to swap ends. I like to think i learned something from that...

 

Just finished installing my Alta 19mm sway on my MCS tonight. I was the lucky guy that won it at the MOTD Welcome Dinner! It took me about 2.5 hours, not including a trip to Sears to buy a 16mm socket and one of those "box" rachet wrenches to go with the other wrenches I already have. (I don't know why, but all the socket sets and metric wrench sets DO NOT include a 16mm. It goes, 14mm, 15mm, 17mm...Can't figure it. 16mm must not be a very popular size!) I currently have it set on the middle hole.

I took a spin around the block to make sure everything sounded OK. So tomorrow I plan to take it on test run. Too bad the Dragon isn't closer.

I think this adjustable sway is the right size for me. A 22mm bar is not necessary unless you are doing some serious track events.