I've found that 450 works beautifully on the gxp but that was with bilsteins, I think the bigger issue also is strut travel. Which can be mitigated with custom hats. I probably have the biggest investment in handling here besides james. My setup now:
Tubular lcas 34mm sway home made coilovers on agx shocks now. Full spherical bearing caster camber plates. Full bmr rear and stb front and rear