I installed reflective foam core garage door insulation and am VERY SATISFIED with the results. (Stuff I used is here:
Double Garage Door Kit – US Energy Products)
My garage doors (3 car garage) are chocolate brown and face east. They catch all of the morning sun, and until I installed insulation radiated inward like pizza ovens. I took careful temperature measurements for several hot summer days before installing and several more hot summer days after installing the insulation.
Temperatures of the inside surfaces of the doors lowered dramatically, easily the 20 degrees mentioned in other posts, and on a few days up to 50 degrees different. HOWEVER, surface temperature of the doors isn't what you're living with when working in the garage. It's the air temperature that's important. So...
Before installing, the air temperature inside the garage would become 7 to 10 degrees above outside air temperature. Afterwards, inside air temperature remained 4 to 6 degrees below outside air temperature. In short, the insulation made noticeable reduction in the pizza oven radiant effect, not 20 degrees, but certainly cooler than the surrounding outside air temperature.
If you're also going to use a mini-split, the insulation will save it some work.