We have one and it doesn't seem to get warm, and LO doesn't seem to get warm from the velour- the velour is mostly around the sides, the main body is fabric.
In the summer, we always use sun shades, leave the windows cracked open, and we blast the AC when we first start the car, and DS has always been fine.
We really like this car seat BTW- he seems very secure in it, and looks a lot more comfortable in it than our Safety 1st car seat.