Filesystem load to RAM vs piping

I must admit I was hoping for something a bit more in the mBps range

Yes, you're not going to get close to that I'm afraid! If you want that kind of thing IMO you really need to be looking at much more heavyweight, linux-based systems.

I believe that page could be quoting bits per second, so at 300kbits you're still only transmitting 30k bytes per second. Given Espruino's having to read from the SD card as well, 8kbytes per second isn't that unreasonable.

what is likely to be the bottleneck here?

The serial comms to the ESP8266 won't be helping at all, but the throughput from the SD card really isn't amazing either. Unfortunately you can't wind the serial baud rate right up because then it's possible that Espruino can miss characters that come in, and it gets unstable.

If I watch the AT debug data, I see 64byte packets being sent

That is strange - did you remove the chunkSize from Pipe again? It should send around 530 byte chunks each time if left to its own accord, so with the chunkSize set to 512 the FAT sector size and the amount of data sent at once should match up pretty well.

It's worth noting that with at.debug on you might well notice some instability/slowdowns - since Espruino is having to relay everything that happens on the serial link over USB, buffers could get full and it might miss something.