Your units were confusing. I assume by 'mil' you are talking about thousandths of an inch.
I have not found Lexan as yet and am using some clear plastic sheet, 2mm (1/8") thick. Lexan is a superior material, but I am not selling lures, or fishing rocky waters, nor sub-zero temperatures, so it will do for now.
My thoughts are that the lip material should be as thin as possible to get the job done. The lips job is to A) deflect the water to the sides and generate the vortices that give the action. Try not to spoil the aesthetics of the finished product.
While regular swimming, the lip must be stiff enough not to bend or distort. The lip must also be strong enough to take a reasonable pounding from submerged rocks. Whether you should have to design the lip to withstand abuse from an idiot, casting onto rocks all day is probably debatable.
As Bob stated, 1/16" is a practical thickness to work with. If you want to use thinner, you need to give it a violent work out to make sure it takes the abuse before selling any lures.
If you compare a thin lip with a thick lip on the same lure body (not at the same time), there will be a difference. Not because of the thickness, but because of the change in weight distribution, average density, centre of gravity, etc. If you are looking for the widest wiggle you can possibly generate, a thin lip would be an advantage, as the inertia of the lips mass is reduced.
Nothing more than just a few of my thoughts on the subject.