Are vias underneath a SMD component without a thermal pad useful for cooling?
Take note of what the data sheet says about thermal resistance: -

The main point is that heat is transferred via the terminals about 2.5 times better than it is transferred via the body. Either way, the data sheet is suggesting that you use 5 mm x 5 mm copper areas for each pin and, if you bring them on vias to the underside to make 4 bigger pads you will be doing your self a bigger favour than just having an unconnected copper area beneath the chip taken on vias to another copper are on the underside. Every bit helps though but concentrate on the pins is my advice.