I didn't quite understand your question with regards to the buffer changing and DMA manipulation of CyUSB.sys.
Which buffer changing are you talking about? on the host side or device side?
I changed device side buffer size up to 48KByte.
and buffer size seems to be set up to 64K-1 byte. Right?
Another question, If an CRC error occured at bulk transfer, do the firmware and PC driver handle this error automatically?