6Mbaud UART with Bus Pirate V4
About the USB limit. 


Data has to packeted into 'legal' packets according to the USB standard. This adds a couple of bytes to each of them, and lowers the speed. 
In between, the packets have to received at the other end, checked, and confirmed with another packet, still lowering the speed. 
The controller in the sniffer will be quite busy receiving the 'sniffed' info, and thus have less time available to attend to the USB port. 
The USB line is also occupied with short polling packets.


That said, 1Mbps guess is probably just to be safe in all circumstances. However, I doubt that you will get to the 6Mbps (or MHz) of the SPI bus reliably.