Quick Dry Foam is available in both soft and firm, same for any outdoor foam. What affect the comfort level of a sofa is primarily the back cushions. The cushions you sit on (seat cushions) should be firm. The back cushions should be soft so you can sink your weight into it.
A good example is Blume Living’s Jibe collection. Its seat cushions are made of high density Quick Dry Foam, which is firmer and more durable than the softer version. Back cushions are made of fluff, like in throw pillows, so they are soft and very comfortable. There is no real need for any throw pillows.
On the other hand, Club collection’s design dictates that both back & seat cushions be made of foam. In this case, adding a few throw pillows will make the sofa more comfortable.