We used a 5/16-5/16 flex coupling on part of our winch this year. In the right application, they are very good. We purchased from McMaster-Carr.
Below is a link to the McMaster part that looks similar to what you are using. it has 5/16" and 1/2" bores. This part is rated for 0.015" parallel offset and 5 degrees angular offset. It sells for $55.
http://www.mcmaster.com/#6208k596/=ryvr6u
Another strong point to make is the connection directly to the CIM. The CIM "bearing" is just a bronze bushing and cannot take axial loading. The motor and connection really needs to be mounted in a way that limits the axial load on the shaft.
Your initial question was if the part from China was as good as a part from other suppliers. I have an opinion, but i don't think this test can validate yes or no because the part was used well beyond what it is designed for.