In my opinion it's good to be open only in a traffic jam or let's say in town at low speed.
When the speed is higher the out coming hot air would struggle with the turbulence. Well, the open lid will make the car less aerodynamic (hm, "less", is that possible in our case).
At high speed behind the car there is plenty of vacuum that pulls out the hot air trough the vents.
Maybe the optimal upgrade is to open the upper half like that: