There are lots of factors to consider here. On the bands where your
antenna is longer than a half wavelength dipole, it will show gain over
the equivalent dipole. If you keep it about 40 feet, it will have a
single main lobe all the way up to 10m, where it's getting close to
being an extended double zepp (two 5/8ths wave halves)

If you make it much longer, the pattern will break up into multiple
lobes on the highest bands (which can, but not always will be, higher
gain, but in weird directions, and there will be deep nulls)

Of course, the higher bands (10, 12, to a certain extent 15) are dead
these days, so maybe we can exclude them from consideration.

- - - - -

The other thing to look at is the matching efficiency on the LOW bands.
The antenna is very, very short for 160 and 80m and is quite short for
40m operation. The tuner, if it will give you a match at all, isn't
going to be operating as efficiently as it could when you're operating
on the lower frequencies. Lengthening the antenna would help that, and
in that case, the gain could change a LOT, in the sense that you'd have
a lot less LOSS.

So it depends on what you want to do. If you're doing a lot on 80m and
40m, and want to have a dipole-like pattern all the way up to 17m, I
would suggest making the antenna maybe 74 feet total or so. This will
still put the 17m main lobe broadside to the antenna like a dipole
(though with some nonnegligible secondary lobes, but that's OK) and
will do much better on 80m as far as the tuner's ability to match.

The multi-lobed nature on 15, 12, and 10 wouldn't ruin all operation
there.

- - - - -

If you want to get gain in a particular direction using just a single
wire, you need phasing sections, but that gain isn't all that useful
unless you have a particular narrow direction that you favor, and your
gain would be single-band anyway.

So what you gain by lengthening a straight wire is more that you'll
have *less loss* on the lower bands, not so much that you'd get gain on
the higher bands (though a 74 foot doublet does have about 2.5dB gain
over a 1/2 wave dipole on 17m), and if you're trying to use a 40 foot
dipole on, say, 80m, you could expect many dB of improvement by moving
to a longer antenna.

Dan