Your test was random read / write of 250 10MB samples. To me, that seems to be how they would be used in a camera. From what I understand though, audio recording is a much more continuous write. I wonder if you can find a test that does sustained writes. I think such tests exist for hard drives, but I'm not sure if they work for flash media.
Testing continuous write proves to be more of a challenge as the tools that I found will give only average data on huge file writing.
The Transcend card tested above (Transcend 32GB, used but not abused, Class 10, 600x, UHS-I bought in 2015) gave average write rates of 34 to 36MB/s when tested for continuous files writing with the f3 program (f3write). So we get the following profile for this card :
- continuous file writing average rate over several GB (seven tests in a row without reformatting between tests) : 34 to 36 MB/s
- random file writing average rate over several GB : 7 to 9 MB/s
- random file writing average rate over 10 MB : less than 1 to 40 MB/s
As I understand it, this card will work fine in a recorder until it meets a situation where writing slows down to a point that it will not provide sufficient throughput for the recorder. When such a situation occurs, the recorder buffer may provide a solution up to a point, however if one supposes that the recorder buffer size is more or less what is needed to provide the pre-record feature (5 seconds on most machines), then very low write rates over 10 MB of data may be a real problem.
Trying to put it simply an average 35MB/s bandwidth is not exactly the same thing if the is no instant when the throughput was less than 30MB/s and when there were times when it was close to 0MB/s.
Summary : average rates on which the card ratings (C10, UHS-I for instance) are based are fine but we may be more interested in worst case scenarios as shown by random file writes rates and we should prefer cards that guarantee a better minimum rate under any circumstances.