lip on MX5 looks dope, wont do shit at low speed, velocity's relationship to downforce increases exponentially by the power of 2 (L = 0.5 x rho x CL x A x V^2 -- V being velocity, L being Lift), so it's not as simple as at 50k's an hour, you will have half the downforce effect of 100k's, hence why F1's are pretty pathetic at low speeds.
but doesnt matter, it looks good, so who gives a shit?