Windshield mount laws are generally not enforced but lets just say you get pulled over. Most likely they will tell you to remove it and drive away without a ticket. If for some odd reason the cop gave you a ticket for it, I am sure the fine will insignificant but probably easily dropped in court. On a side note, oddly enough mounting it on your dash is legal in all those states but a dash mount blocks more of my vision then where I mount it on my windshield. Luckily it is not illegal in NC but it is in SC and Ga (where I am from) and I was never aware of it until now.
Catalytic converter is a much different animal. First off, it is a federal law that was passed in 1988 with the EPA Clean Air Act and carries much higher penalties.
Fines for the owner can be up to $2500 and for the institute that removes it, up to $25,000
Chances are you won't get caught unless there is an inspection in your state.
However, is it really worth the risks and headaches for something that won't lead to much gains? Even if you gain anything at all, any notice to those gains would be purely placebo.
Here is the law if anyone is actually interested in reading it.
Title II | Clean Air Act | US EPA