Hardware point of view
As you are using a FPGA, surely you will be able to use the high troughput available in standar mmcsd bus. Your question depends of many factors:
- Which bus are you using, 1bit mmcsd, 4bit mmcsd, 8bit mmcsd or SPI.
- Which is the frequency that you are clocking the data in the bus.
- Which version speed is your SD card, classA, UHS...
Software point of view
If you are using an OS (e.g. Linux) probably you are using a system file, hence the write/read throughput depends of many factors.
- Which file system are you using, FAT, FAT32, NTFS, EXT4
- What are you trying to transfer and from which place is coming (this affect more to write operation).
So As you can see, there are many factors that can give you a exact throughput. My personal advice is find a benchmark related to SD card in FPGA and use the worst case (lower speed) as starting point for your design.