SPI and UART are totally independent interfaces on the ESP8266 devices. Also the max clock speed for the SPI interface is the system clock/2 (40MHz?) but that would probably imply the need for DMA to keep up with the value exchange rate.
HOWEVER, 2MByte/sec means 16Mbps. Even though 802.11g has a theoretical transfer rate of 54Mbps, you need to take into account the overheads involved with the protocols used to encapsulate the your data, plus the 'interference' that other WiFi traffic will cause. Then you need to take into account what else the two end devices are doing. Your FPGA device may be dedicated, but your android phone may have trouble if you are after a sustained transfer rate that high.
Susan