Sunshades keep the sun off the dash and upholstery, but used on the interior of the windshield, they probably don’t keep the interior any cooler. I live in a hot climate and use sunshades to protect the interior, and keep the windows down about an inch, but it still gets very hot inside the car.
BTW, sunshades actually work better on the outside of the windshield. Use the wipers to hold them in place. The problem with this is that they tend to get stolen unless you are in a very secure parking lot.