How much will adding 16 lb to my car affect ET/mph(rotating weight)

I think it depends on where the extra weight is. If it is closer to the axle center line (heavier wheels) it matters less. The farther out it is (heavier tires) the more difference it makes. The heavier the weight, and larger radius, the more energy it takes to accelerate it.

I believe what you are talking about is centrifugal force. For a fixed radius, the force increases proportionally with the weight and increases with the square of the wheel speed. More of your horsepower is stolen as the centrifugal force goes up.

I believe this is the general path you would need to follow to get a real answer. If you could figure out the weight distribution across the radius of your wheel and tire and a wheel speed of interest, you might be able to come up with some estimates, but I don’t know how to do that.

I am guessing that the easy way to do this is just try it, as the math and measurements you would need to figure it out look complex.