I recently purchased a 98 AU XR6HP, and agree the idle is a bit rough. I checked a couple of other examples on car yards, most with higher km's (mine is 36,000) and they all seem the same.
But when it comes down to it, as long as it does'nt flame out at the lights and runs fine at speed, who cares.
I rented an AU3 forte up in NT last week, idled much smoother, but was much wheezer than the XR, real glad I bought the XR. The forte seemed to run smoother on the long roads, less suspension crashing and thumping due to higher profile tyres and more complient suspension, but did'nt have the same footholding and way to much body roll for my liking. It did'nt mind doing 1200km at an average of 140km/h, cool, rev limiter a bit of a pain, again glad I got the XR.