Adjustable rear sway bar for either G or H (same part) would be $238.50.

The other thing I'd highly recommend for both is taking the slack out of the front bar with heavy duty links:
http://www.whiteline.com.au/product_..._number=KLC154

This has the effect of increasing response without loosing grip (which is the normal sway bar trade off). $161.10 for the pair, again, for both Astra G and H.