Guessing 30" in diameter, it would only take about 3.5 psig overpressure to lift one at 200 lb weight if my arithmetic is correct. Less if they're bigger in diameter than that.
Your math is wrong. 30" diameter = 225pi square inches (706), so 0.28psi for 200lbs.
Tested and failed by the Mythbusters. Not even a F1 racer can actually pop the cover. They weld them more out of paranoia.
Yes, not because the average delta pressure across the car isn't enough (given an F-1 car is about 4.6x1.8m, and about 75% of that at least is downforce generating, that's ~10,000sqin, and 3+g of downforce in a 1550lb car is an -average- downforce of 0.45psi, more than enough to lift a man-hole cover, with ~0.5g of excess, which, given about 3" of acceleration room, or theoretically, enough to achieve about 6" of peak height)
It doesn't lift because the 0.45psi isn't the relative pressure under the car, it's the difference between the static pressure on top and on the bottom, and the bulk of it is an increase on the top, rather than a decrease on the bottom, hence, no manhole cover lift.
The is somewhat intuitive given the camber on the wings--steeply cambered wings have an effective increase in pressure on the concave side that is much greater than the decrease on the convex--as a pressure increase in a concave side is stable, while a decrease on the convex side can stall/have massive boundary layer separation.
Further side note, since the man-hole would have to clear the car if it did achieve lift-off, the car would have to be moving at least 40mph for 3" of clearance, or 120mph for 1". So given that the maximum downforce occurs at high speed, even for extreme cases, if it did lift, it wouldn't hit the car.
Even further side note--because the pressure aspect is reversed on a sucker-car, a fully skirted car with or without an active drive fan -could- suck a manhole cover up.
Even further...given a thockness of ~1", a manhole cover has an areal mass of 0.29psi...so that is the value to lift it, regardless of diameter.
Even even further...a wind-speed of 140mph creates 0.29psi of stagnation pressure--and if it caps a tube, and the incoming stream is 1/2 the area, only 100mph is needed, meaning 4 3" diameter leaf blowers, each with a speed of 140mph could blow a 12" manhole cover off from underneath.
Okay, I'm leaving this rabbit hole.