Of course, there is another option, that it is only a 1MB board and the seller made a typo. I suppose that is possible, in which case I'm not that torn up about utilizing it. But it would be pretty cool to have a 286 with 10MB.

A memory in an expansion board will bea (as much) as fast as the bus speed. That's in a strange situation where there is no other device (like a video card) trying to access the bus at the same time, so in the real world you won't get that speed.

Also, ISA speeds were up to 10 Mhz, so I guess that any processor will have to wait some cycles before getting data on expansion boards.

The good news are that "some cycles" will always be better than wait for the HDD to serve data. That RAM will be slower but much better than not having anything, and that RAM will be usable even on 386. But remember that most 386 board accept more memory on board, so 386 will get much faster if you simply fill up the mainboard memory slots before adding an EMS card.

So this would ultimately be much faster/more useful in a 286 class system?