I think the 10MB/sec claim is a sort-of max-speed-on-the-fly rating, probably on a freshly-formatted card. When the cam closes a file (for shutdown or prior to opening the next file) there is a heap of system read/writes needed to update directory entries and FAT tables for the operating system. Once you have initially filled the freshly-formatted card, issues of reclaiming used space further complicate things.
I don’t get any corrupted files when I power-off, just a picture of my palm over the lens as I hold down the button for a second or two.
My current card is a 32GB Sandisk Extreme Pro. It claims “up to 100MB/sec Read and up to 90MB/sec Write”. More relevant are the V30, U-3, and A1 ratings. Basically, you gets what you pays for.
The “high endurance” cards are old-school - class 10 with “up to 20MB/sec Read and Write” and it is fine in my car (which spat out a Sandisk Extreme after a month or so) - so wait and see.