Yes it is known, and it is not constant in any airline environment. No airline I know of these days flies around constant Mach, its all econ speed. The cost index in the econ speed takes into account the crew costs. A 787/A350 in one airline will run around at different speeds to another, it all depends on what the accountants desire, not the pilots. The A350 will happily go between 0.76 and 0.96 and burn about the same fuel as an A330.
Thanks for all the info, SWH. I'm aware of all the variables with Cost Index but as fuel is the most significant cost, we find that an aircraft type is operated at similar speeds by different airlines. What I was asking was what speed the A350 was designed to realistically cruise at when it is at optimum altitude for its weight and I believe Veruka Salt's answered the question. Wing sweep is probably the most significant of the many variables that affect the efficient cruise speed and, in the past, Airbus widebody's have had less wing sweep than their Boeing counterparts. It appears that with the A350 they realised the need to cruise faster than M0.81.