My 67” APR wing (considerably larger than the MSO one) provides a lot more stability on track (I cofounded a track day company, so I am at the track quite a bit). On street, yes, never going to be going fast enough to make a difference, but depending on the track I’m easily pushing through many corners at 100 to 120mph (at Willow Springs I hit 140mph+ through turn 8, before getting to the decreasing radius of turn 9), especially sweepers where downforce helps a lot. Before and after tests definitely show a greatly diminished tendency to break loose (my previous R888R, and current TrofeoR, tire compounds help too, as does my slightly wider front/rear tread width from stock, to reduce any unwanted understeer/oversteer tendencies).
The 570 was designed to break loose to make it more playful, so I feel that the typical rule of increase front and rear downforce at the same time is a bit different here. Most cars are not designed by the factory to be tail happy - they are striving for lap times, and thus have designed the aero as such. On the 570 though they didn’t do that - so adding considerably more rear downforce is just going to settle it more, not throw the aero balance completely out of wack causing crazy understeer.
On any other McLaren, which are all designed for precision lap times, I’d likely agree (I haven’t tested or researched other models, so won’t assume) - but my experience on the 570 platform says different.
The MSO wing kinda proves my point a bit too - as the proponents of “OEM knows best” say don’t mess with the factory aero (or factory anything for that matter), then why does the MSO wing only largely affect rear downforce? They wouldn’t give you a $13K upgrade to upset the track performance of your supercar, especially coming from McLaren. Also, the guide vanes that come in the kit are not there to increase front downforce to “balance” the wing - they are there to smooth airflow to further reduce underfloor drag - so essentially McLaren thought at least an additional 75KG of rear downforce was needed to improve track performance. Mine provides more than that, at least in CFD, but I plan to take direct suspension measurements as soon as I can with damper pots, so I can confirm how aggressive I can adjust the AoA without hitting the aerofoil stall speed based on the overall aero package.