CyUSB.sys is high speed capable driver. But maximum bandwidth possible with Isochronous transfer is 24 MBps (3K Bytes in one microframe). What is the no: of packets/microframe that you have specified in the endpoint descripotr, and what is the endpoint size chosen?
Please make sure that you are using the ISO transfers with multi packets per each microframe.
Also, I would recommend you to use Endpoint 2 so that you can configure it to use the entire 4K endpoint memory. (size=1K, quad buffer).