Without going a semi-slick (and not many of these are road legal - grey area) I think I have the best tyres I can get in the size. Grip levels are good now. The rear certainly twists. Had the same problem with my first GTVi years ago. Some people I see have fitted rear sway bars which would help but its not allowed in my class. The King Springs are fairly stiff. I looked at both Pedders and Kings but Pedders don’t manufacturer rear springs anymore for the Sirion, just fronts.
Exhaust is completely free as long as it remains road legal (there is a sound limit in place, you trigger the sensor you are DQ for that session). Anyone know who does a set of headers for K3-VE2 in Oz? High flow cat might be on the cards. I wasn’t sure how restricted these exhausts were.