You could get the Opel GT pads they are suppose to be a little better than the Solstice/Sky pads.
If you have the original OEM tires a better improvement would be a set of 4 Ultra high performance tires.
Also upgrade to the something like ATE Superblue brake fluid.
Very happy with the Porterfield pads, DDM Works sells them among others, I got the SP performance rotors as well and I test them daily on 12 miles of twisties on the way to work. I have been running the ATE super blue like LV mentions for a long time, you should have your entire system purged at least every 50K (ALL brake fluid absorbs water), the reason the ATE is blue is so you can see the color change when you bleed the brakes to know the old stuff is out. When I do it next time I will use the ATE clear to see the change back to clear, a simple idea that works.