Check out
www.whiteline.com.au for what look like superb rear anti roll bar kits for the 147 GTA.
They have 18 and 20mm diammeter variants,plus am adjustable rose jointed droplink setup option.
Apparently in general 147 GTA ARB kits can be made to work for the 156 GTA, so I might investigate this kit for my car, with 20mm diameter rear.