Oh boy !!!!!!! Time to sort out the TWO different Z0K's !!!!!! And other stuff.
In no particular order;
There really are not a lot of choices of sway bars for the Kappa platform. It is reasonable to expect that there won't be either as the production run was low (probably under 100,000 in total) and as outfits such as Energy Suspension, Eibach, and Koni have discovered, demand has been rather low. Beyond the parts available from GM, it appears that only Eibach has the only after market bars. These bars were rolled out very early (before the GXP/RL was put on the market) and are upgrades from the FE2 bars. Nobody supplies bar rates but Eibach did say that they are about the same "rates" as the FE3 bars. Nobody except GM offers (other than custom) stiffer bars than the FE3 units.
Five (maybe six) different packages from GM for the Kappas (all developed for street or road race, and in typical GM fashion they all lean towards understeer);
1) FE2 - standard on the 2.4s. Compared to the FE3 set up, the springs and bars are about 18-20% softer, and the Bilstiens are valved to them. Bars are both hollow and (IIRC) 28mm front and 25mm rear.
2) FE3 - standard on GXP/RL. The front springs are 175 pounds per inch and rears are 220 pounds per inch. The bars are hollow and 33f/29r. Bilsteins valved to the springs and bars.
3) ZOK for the 2.4. Basically this is the FE3 with shorter springs.
4) Z0K for the GXP. Roughly 18-20% stiffer (and shorter) springs and bars, and not sure about how the Bilstiens are valved. Also, a beefed up rear chassis cross member.
5) GMPP handling package (early package for the 2.4). It is roughly the FE3 package for the 2.4s with marginally shorter springs. Absolutely not worth a dime if installed on a GXP/RL.
6) GMPP handling package for the 2.0. Some on the Solstice forum say this exists but may be nothing more than some of the GXP/Z0K stuff.
EVERYTHING bolts up to ANYTHING. It's all interchangeable. Koni offers over the counter single adjustable shocks for the Kappa. If you want ONE mod that will improve the way your Kappa feels, rides, and handles, this $600 set of shocks is simply the best money you can spend. In every way they are better than the Bilstiens. KW, BC, HKS, and maybe a couple other outfits offer replacement coil overs for the Kappas and offer different solutions from $1000 to $2000. For those with DEEPER pockets there are custom DA Konis, Penskes, Ohlins, AST, and some other shocks. Beyond bragging rights, these are geared towards racing. GM springs top out at about 210f/265r. There are infinite spring rate choices in 2.5"/65mm diameters. The Konis will need sleeves and upper spring adapters for this, and all the other above after market shocks use 2.5" springs. Currently I run 400f/450r (soon to change a bit) with SA Konis set rather firm and that may be the practical limit for daily use unless you like a really hard ride. The ProBeam and BackBone WORK, and with firm suspension the BackBone will reduce the cowl shake the stiffer stuff will increase. Kappas flex. Stiff suspension will make Kappas flex MORE. Some more negative will not really hurt the tires but sure will improve the response of the Kappa, not matter what is underneath it. As I have played with shocks, springs, bars, alignment, and other stuff over the past three years as I have tried to improve the handling for first SCCA Solo Stock class, and now Street Prepared, I marvel at how responsive the platform is to changes. Budget willing 2010 will see 18X9 or 10 wheels with 315/30X18 Hoosiers for racing and better shocks which should be incredible.
Depending on how you want your Kappa to ride, handle, and feel, with a switch to better shocks and some mix and match of the GM springs and bars you can make your Skystice be whatever you want, and for a lot less money than you might think. HTH.
