Yeah, 850VA input is quite reasonable. It has PFC, so it's not drawing, say, 1.2kVA for 600W, which is quite nice!
It may be that it's rated for 600W, but goes up to ~800W (plus losses) when operated near or beyond current limiting conditions, at certain input voltages. It is probably not 2kVA (at 240V), but if it is, that would be important just to be sure that, in case it needs it, you can deliver it safely.
Input current rating is only a maximum figure, for wiring and fusing purposes. It is not indicative of actual operating current or power draw.
Tim