It is necessary to understand the flow of the cooling air before changing it. As teh Abarth models often had the lid propped open, I'm assuming someone did their homework a long time ago.
On a VW Beetle, the air is taken in through the vents on the rear pillars, or the lid, wherever, it changed a bit during the lifetime of the model. The air is then pulled over the top of the engine, and pushed down under, and over the cylinders by the fan. It is important that when the lid is opened, there are no visible gaps in the tray that seals the engine upper. Even gaps around the plug leads can cause issues.
Any gaps allow the hot air to re-enter the engine compartment, re-circulating, so getting hotter. Surprisingly, those air-cooled engines are susceptible to overheating issues if not sealed properly.
So, with the 500, you need to follow the air flow, to ensure that opening the lid does not allow hot air to re-enter the circuit.
The standard vents should be sufficient to allow enough air to circulate. It is doubtful that Fiat would have created a restriction by not having enough or not large enough. Ultimately, airflow is created by the fan, drawing from a bigger intake 'hole' will not create more flow.
But they do look a bit mean with it propped open.