Is 10AWG copper wire sufficient

To drive my Usher CP-8871? I will be biamping it using Lexicon amps.
Unless you also need to moor an aircraft carrier with it as well, yes.
How many feet long are your cables? 10 Ga. wire is awful heavy, and probably not necessary for the sake of current delivery for normal length runs.
10 gauge is a great size. I had 12 gauge and upgraded to 10 gauge with 4 wires in it, so I doubled up on each spade. It's basically 8 gauge going to each terminal now. There was a definite big improvement in all areas.
These size wires are good for power cables, but not really necessary for speakers.
I think you would be prudent to star with garden-variety 10 or 12 gauge cable, such as the stuff available from Blue Jeans Cable. Listen to it for a year and then you'll have a good point of reference as to whether you hear any meaningful improvements from something more expensive. In short, plain wire is fine.
I would think something like 16 gauge would be sufficient. Having multiple strands that would sum up to 10 gauge is quite different from a single strand of 10 gauge.

All the best,
Never understood the whole AWG system. Why not use wire diameter or wire surface area in mm or mm2? No one outside the US knows what AWG is.