...And it loads up nicely SWR-wise as most of the power is dissipated as heat in the coax...
These two conditions go together only if you have excessive loss in your coax
to start with.
If you use a remote tuner at the antenna feedpoint, the coax losses will be low.
That might be an auto-tuner, or a manually-switched tuner, or even a tuner in
the shack if the feedline is very short. In the case as originally described, where
the antenna is matched with a wide range tuner, the main source of loss will be
in the ground system unless there are enough radials to make a good ground.
The main performance limitation above 40m is due to the low antenna height.
Depending on the exact length (in wavelengths) on each band, in some cases
the vertical wire will be responsible for most of the useful radiation.
On 40m the antenna performance will depend on the ionospheric conditions
and where the band is open. On 80m the antenna will work reasonably well
for local stations, but the ground losses will be important.
In any case, you need a good ground system to feed it against, and if the
tuner is close to the feedpoint the coax losses will probably be insignificant.