Ok - now you know the facts - here are the questions:

1) A friend recommended I elevate the feedpoint only to 32'
because (he says) it'll work better at a multiple of a 1/4
wavelength. Is this true?

NO...doesn't matter.

At the heights you are talking about on 40m (40'), the higher
the better.

2) What can I expect in performance improvement at either
32' or 40' off the ground? The house is all wood construction
with an asphalt shingle roof.

Either will be a big improvement over your current 20', but
two 10' masts might not be stable enough without some guying.

Tor
N4OGW