Pretty sure this has been quoted since the 90's and it's true. But the car would have to get up to speed the right way first, obviously, and then have a spiral track gradual enough.
A quick google throws up some similar race car data quoted here: http://www.ultimatecarpage.com/forum/showthread.php?t=41451
An f1 car weighs about 1500lbs so at 150mph we could pretty confidently assume that one would have twice it's weight in downforce at 150mph-ish.
Using the square law for lift then that would give about 70mph for the break even point, obviously you would need to exceed that by a safety margin. Of course you could also set up the wings in Monaco high downforce mode to get a better CL etc.