Rear sway bar upgrade?
 

I believe you are thinking about this backwards. You would be better off going with a larger front sway bar to balance the car overall. In SPC, most of us run the Tarett 19.1mm hollow bar. Even though it is basically the same diameter as the stock, it has a higher spring rate. We balance that with the 28.6mm GT front sway bar. All that said, we all run the bars full stiff front (or half a hole from full) and full soft rear bar. Some of the guys run the stock rear bar because they want it even softer. Of course, you also have to take into account what suspension you have, ride height and tires, but, in general, I think you should be looking at the front of the car not the rear for balance based on what you have said.

A stock 987 understeers. Stiffening the rear bar will help this and make the car more neutral. The rear bar is easy to install. That said, stiffening both ends will help the car. I used the adjustable GT3 front bar and Tarett adjustable rear bar which allows fine tuning to your style.

An Anti sway bar REDUCES grip, it does not increase grip. More rear bar makes the rear more loose. More front bar makes the front more loose. Anti sway bars are a compromise item, they give a little, but at a cost of something else. In a perfect design, you would not have either front or rear anti sway bars. The larger the bar, the less independent the wheels become. More bar reduces independent movement at the wheels. You want more rear grip? Try removing the rear bar for a day. I did. You will be surprised at the results. (Not surprised like "Oh S*@^%, the rear end is out of control.) You might find that you prefer the car with no rear bar. BTW, Front and rear factory bars are designed to make the car understeer. The lawyers want the car to crash front first since that is where all the crash safety bits are located.

https://cimg1.ibsrv.net/gimg/rennlis...63d93ce7ec.jpg
This may help - it is a good cheat sheet.

^^^^^^^ I keep that in my setup book... As much as I think I have it memorized, I still go back to that cheat sheet..

Great advice!