Mine does the same, but only when cold, idles between 1200 and 500 rpm, goes up and down, but never stalls, and usually after about 30secs it sorts itself out and everything is fine, I just thought it was a bad tune on the "cold" setting on the power-fc?
Also for avg fuel economy do a search for the "How much Petrol does your stagea use" thread. It really all depends on how much throttle you use, I can squeeze ~600kms out of a tank if I am careful, then again with a heavy use of the throttle i have gone as low as 250-300kms out of a tank, all depends on how I drive.
I highly doubt that just using 95 ron fuel would make a difference with idling, maybe make it ping a lot more underload, but not idling.......?