it cant have the same read and write speed. you are watching numbers when the files are cached to the internal ssd and later written slower to the tardisk card.
once you start using Tardisk, you cant eject it. it has no native os level support, so you may end up having a corrupted filesystem on your hands. The system may eject the card for example to save the battery or whatever reason.
as it works similarly like a fusiondrive, it still is just an external card. i wouldnt start messing with CoreStorage and volumes dividing them to two differend physical storages and one not having a native support... Too risky to loose data.
as it sounds nice to double the storage so easily, it is far away beeing so simple solution.
I looked at their site. The SD card looks like it would probably be fine. It can't be faster than the internal SSD though, as USB 3 that the SD card is plugged into is slower than SATA and PCIe. Also, there are no SD cards available that are as fast as a decent SSD. There's also no way to create a fusion drive without reformatting. So, it's either a custom driver or they're just sticking the drives together and telling OSX that it's the same drive. If I bought one of those cards, I wouldn't use their software and just symbolic link a few big directories to it, like iTunes.