using a pcb pad for a heatsink is only good up to about 1 sq in, plus it helps to remove the solder mask and leave the copper exposed to air. We use thermals vias and many layers with as much copper connected to the thermal vias as possible, I'm talking like 8 layers. Plus thermal pads connected to the bottom side and transferring the heat to the box, we're able to push 40-60W through our boards this way.
To get full use of these parts, you really need to add a heatsink. They give you the thermal resistance junction to Ambient, then you can decide how much power you can push through a part just as it is. If you want to go the full monty, you have to add heatsink/air flow. This is true for many parts, especially power diodes and regulators. The 7805 can do up to an amp, but needing 1.5V overhead x 1A = 1.5W, and you can't push that through a TO-220 without a heatsink + airflow. You're lucky to get 200mA through it w/o one.