Would it be worth the extra effort to try to get the base of the antenna
up to 40 feet rather than 25 feet?



Yes and no... Will help 40 a lot. 80 some.. 160, not a whole heck of a
lot.. 40 ft is still low in terms of wavelength on 160. Say you had a
ground mount with 60 radials.. To equal that ground loss with
four radials will require the antenna to be at about 1/4 wave height.
_____________

I know of commercial AM broadcast stations using 6-8, ~1/4-wave radials
elevated less than 20 feet over (rocky) ground, with an antenna system
radiation efficiency meeting the FCC minimum for broadcast station use.

Also this quoted conclusion hasn't been supported by the NEC evaluations of
L.B. Cebik as given in the paper linked below. Figure 16 in that paper
shows that the gain of a vertical radiator is within tenths of a decibel of
its peak maximum value with four radials each 1/4-wave long, when the whole
system is elevated only 0.075 wavelengths above the earth (about 41 feet at
1.8 MHz).

http://www.cebik.com/fdim/fdim4.html

RF