A typical maximum clock speed number auf regular SD cards is e.g. 25 MHz, which means, that 4 MB/s can be only written in 4-Bit SD mode. Newer extra fast cards support clock speeds around 100 MHz and should be able to achieve the intended speed also in SPI mode. The speed capabilities of a specific card can be read from CSD register.
Data rates are primarly burst transmission rates, to reach it in sustained write, additional requirements have to be met, particularly writing full physical memory blocks by multi-sector write commands. I don't know however, if the existing Microblaze SD card driver will support optimized multi-sector writes.