Any tire with the correct outside diameter and load capacity (not too wide, etc) will be OK. The 20" Ziex CT50 A/S has an O.D. of 30.1" I think a variation of no more than 2% is OK. The load index is 104.
255/50-20, 255/55-19, 255/60-18, and 255/65-17 should all work well. I'm glad the myth of a narrower tire offering better grip in snow has been put to rest. The tire contact patch is the same regardless of width if the inflation pressure and load are the same. Continental says,
http://www.continental-tires.com/car/technology/wide-tires
The Bridgestone Blizzak DM-V2 tends to be a heavy tire. I like the V1 a lot on my pickup truck. The Michelin Latitude X-Ice Xi2 and the Nokian Hakkapeliitta R2 SUV are both lighter weight and also excellent in winter. The heavier rotating weight is harder to get turning and harder to get stopped, as well as causing more inertia in the suspension for a less-smooth ride.
I'm thinking of the Nokians on a set of Sorento wheels I have, 17x7, 5x114.3 bolt circle, 48 mm offset, 67.1 mm center bore. They fit without interference. I found one web site that lists the CX9 offset as 45 mm. 7" is the minimum allowable wheel width for 255 tires.
I'm surprised the RX8 wheels have that much offset. I
think they'd be OK on the CX9, but it is always good to do a trial fit to check for interferences.