the GXP bonneville has a strut tower brace that comes stock if you can find one in the junk yard it would be cheaper than goin to gm not sure of the part# or price from gm nor where to get one aftermarket
as far as suspension they dont have much of anything that i have found yet however go to rockauto.com and they will have the kyb struts and the max airs for pretty reasonable also there is a discount code for us to use in the lounge i believe, that comes in handy when cash is short.
our cars are different cause of the air suspension in the back so suspension will be difficult.
i have the max airs and the do very well in cornering and holding the rear up when ya punch it the kyb struts i hear are very good upgrade for those of us that like to corner fast and basically drive like nascar worth the money
2005 Pontiac Bonneville GXP
2008 Chevy Impala 3LT 3.9L (9C3)
2000 SSXP (retired) all over the country in a million pieces