So let's assume you will be riding on 18s. The ride quality degrades a bit from 16s to 17s but not much from 17s to 18s. The must have mod then would be to get front and rear sway bars. Since you are not looking for the absolute handling sways (TRD Racing blue), I would highly recommend the Daizen Sport Tuning sways. I have run both. The Daizens are maybe 95% of the TRDs in terms of handling crispness but the ride quality does not really change with the Daizens while you will be tossed around a bit with the TRDs. Try this to see if it is enough for you. Also, handling and ride quality is also determined by the shocks and springs you have. Do you know what shocks and springs are on your car right now?
There are no camber kits for the GS. You can wait on the STB until you see if the sways are good enough. The steering ECU helps tighten the steering at highway speed but not much at all at low speeds. The feel is somewhat subtle though. It is not night and day so YMMV.
Do you want more power or for the car to respond better? For daily driving, the GS400 has ample power. You can get a better response off the line by adding a high stall Torque Converter and a Limited Slip Differential. With these, you will not get more power, just more efficient power and take off at the line.