I've done some searching as well, with a lot of heated discussion found. In one thread, several people were adamant that if one already had more than 2gb of conventional RAM, that ReadyBoost wouldn't provide any significant benefit. Others disagreed.
Here's perhaps a dumb question on this topic:
If one runs out of conventional RAM, the OS has to turn to a SwapFile. In the case of the Surface 3, that would be the only drive it has, which is an eMMC. Although not as fast as a conventional SSD, wouldn't this still be quicker than a MicroSD card? If it is faster, then what would be gained by using ReadyBoost on the MicroSD? If the MicroSD is faster, then perhaps we're talking some potential benefit.
Theoretically ReadyBoost will self-enable if it identifies a drive that could provide a beneficial impact on system performance. The fact that it is not enabled on my MicroSD might suggest that it wouldn't provide any benefit.
Interesting topic, in any case.