How much more efficient are the 40 plate chillers compared to their 30 plate peers?
I do not foresee going over 10 gallon batches.
Is the 40 plate overkill for the homebrewer in small batches?
there's a great chart for home brewers up on DudaDiesel: http://www.dudadiesel.com/files/wortchart.pdf
to answer your question directly, it depends on how fast your coolant is flowing, but the difference doesn't look to be that significant to me.
in that chart you should also note the model of the chiller, which between models w. the same number of plates has to do with the overall length of the unit. longer seems better.