I'm not sure, would street tyres with a softer sidewall be better? Maybe even drag radials. I don't think anyone has done any research on AWD cars (probably the EVO / WRX boys), but those Hankook's are so stiff that - 10PSI doesn't seem to have any effect.
We just got some Performo 105 ELF fuel to try, we have only run the WRF so far, so there is more room in there, but damn, its expensive!