I only have a piece of anecdata, and it relates to a proper high mount wing. The wing was an s1223 profile, about 1.8m span and 300mm chord, mounted at the rear of an MX5 racecar behind the boot at roof level. The car was trailered to the circuit, no ill effects noted. On the return trip, the combination was plain evil, wandering around at 110-115kmh virtually unmanageably. Checking tyres, load distribution no problems identified, on resuming behaviour unchanged (unsurprisingly), and speed reduced to more moderate levels to rein in the instability.
The penny dropped when we realised that the tow was into a strong headwind (tailwind on the way down), and the downforce on the rear of the car was enough to decrease the towball weight enough to affect stability. After that the wing was removed for towing.
A friend towing a Sprintcar with a big wing on an open trailer [30+ years ago] used to have the l/r tyres occasionally scrubbing on the mudguards. And the poor old F truck used LOTS of fuel to tow it. Though it does prove the wing was giving inside weight,,, it was a 2 ton trailer!!
Towing my now 50 y/0 Supermodified around with the wing off is a noticeable improvement, even with the wing flat it is giving downforce and lots of drag!! On occasion I will move the car forward about 100mm to stop the downforce effecting the trailer.
It is amazing that on a 2.5 load rocker sprung tandem trailer with about a ton of racecar and spares on board.
And when I added a wing on my Sports Sedan I learnt how loose the car was without it. What I gained in cornering I lost half of in extra drag. But the car was so much more stable, especially over 130mph. Never really noticed much at towing speeds, if any thing it seemed better with the wing at roof level instead of about a foot lower. Higher it was better at race speeds however
As for boot handles on road cars,,, well it looks faster!! And you need heavier boot struts to hold the boot lid up!
As Greg has said modern cars have a bit less lift meaning a bit more stability at the front. The sporty ones often have less as they have knocked them all off. A friend with a modern V8 Commodore with all the wings and splitters etc says he really does notice it missing at 110k and has had it flapping at over 100mph. It has been replaced at mega bucks about 5 times now. He lives in the hills with spoon drains and steep driveways.
Though even many mundane family hacks drag the front in that scenario. VT on Commodores especially though I have done similar in Falcons and Mitsis and even Oh what a snooze cars
Edited by Lee Nicolle, 14 December 2018 - 12:57.