I'd ague against more than an octave for another reason, pattern.
That and for 12, 10M and 6M up antennas get small enough there are a
a whole other set possible improvements. I'd think if one can get a
monster wire up then several smaller antennas are possible. A
dipole for 6M in wire is maybe 115 inches, no excuses there.
Most do all wires tend to be among the greatest compromises.
FYI even though I have a LNR- quad (40/20/15/10) I don't use it on 15 or 10.
Its the goto for 40 and 20. The K6STI loop on 10 is way better. A larger
scaled version for 15 is very good. I used a 6M scaled version in a tall oak
for FD and worked a lot stuff and had a 3 element beam to compare to.
Beam was better only when pointed that way. Rest of the bands
(not VHF) if I want to work them I put up a monoband wire using
one favorite tree with a lanyard in it, poles, or green mil aluminum
pipe as a temporary tower.
Goes back to the first statement, pattern, you have to hear them first.