I have got my KE55 to handle very well. It corners like its on rails. It seams that the limiting factor is how tightly I can hold onto the steering wheel! I kid you not!
And this is without negative camber. I had negative camber before, and that is one of the good things to do. But I did many other things also, and as it happens my car currently has roughtly zero camber, and it still corners quite well. My specs:
325lb/in front coilover springs
22mm swaybar
all polyurathane bushes
2 degrees caster
Monroe GT shocks (short stroke, to suit ST162 Celica)
XT130 struts
Bridgestone Potenza RE711 tyres
195/60/14 tyres
14 x 6.5" wheels
strut brace
modifyed steering knuckles (don't do this at home)
I would say the most important things are the tyres, and the strut brace.
Previously, I got negative camber by lengthening the lower control arms by about 15mm each. I will have adjustable strut tops soon and am using standard control arms again.