There are no real upper bounds specific to the OT, because the faster cards can also be used at slower speeds.
A card like the SANDISK EXTREME can support up to 800x (120MB/s), but the OT probably doesn’t use it at that speed. The only situation where you may experience the additional speed is during file transfers in an external reader.
I know that it won’t help you in your situation, but just for clarifications: the SANDISK EXTREME models (up to 64GB) gets used by a lot of OT users including myself since years without any troubles. There is just a single report of one not working here, but with no followup what was wrong.
Maybe you got some fake/counterfeit SANDISK EXTREME cards? Since your computer recognizes them I would at least try to fill them with data and try to read the data back (just to make sure).