” It will only make your rig see 50 Ohms and prevent it from cutting back the output power.”
” It will not make the antenna any better, nor will it improve the SWR on the line ”
Although all the above is true, this does NOT imply that whenever an antenna is not matched to the line, power will be lost. In fact the power reflected from the antenna will be re-reflected by the tuner and adds up with the output power of the rig. The forward power therefore is always higher then the output power of the rig. This may seem strange to most people, but it can easily be shown by measuring forward and reflected power at the tuner output.
If you think this is all nonsence please read Walter Maxwell’s Another look at Reflections
On a loss less line no power will be lost. On practical lines, a higher SWR also means a higher line loss, since reflected power travels back and forth.
If this is a problem, one should consider using open line, with a relatively high characteristic impedance.
Since matched line loss is inversely proportional to the characteristic impedance (on HF) 450 Ohm slotted line has about 10 times lower loss than a standard grade 50 Ohm coax.