1) The Apple Ethernet NB card has a 16-bit bus, and is comparable in performance to the older EtherTalk NB card.
2) Very few tasks are downloaded to the card (nothing on top of A/ROSE). So the on board 68010 mostly manages the DMA, off-loading little of the cycles and work required of the Macintosh's main logic board processor.
Incidentally, the SONIC chip used in the Ethernet NB card is 32-bit capable, and when used in Quadra systems is in 32-bit mode.