Observing effect of household appliances on powerline communication
For a reduction in data rate due to interference that is as slight as that shown in your table you are very unlikely to see any deterioration of the spectrum whatsoever.


  I am wondering what would be a good way to quantify this effect.


Do exactly what you did in the table - plot the effective data rate when an appliance is on compared to when it is off - the table tells you a lot so well done on that - it's useful to see. You could presume that the effective data rate reduction is due to packet retransmission due to packet corruptions. You could also assume that when data rates are higher, the packet length is shortened in order to make retries of smaller packets more liable to be received correctly - this also reduces overall data rate bearing in mind that the reduction would be greater if the packet length were kept bigger because of the probability of errors due to longer packets.