Too thin a PCB track will just run hot and waste power. Here is a good graph, taking into account copper thickness and track thickness and temperature rise: http://www.apcircuits.com/ click → Technology & Resources → Amperage chart–> shows 12A, 1oz Cu, 20C rise, use 0.18" wide trace. Polygon pours will work fine.
For the switcher IC there are two main current loops - inductor charge (top fet on), inductor discharge (bottom fet on). The third current loop is the inductor dumping into the output capacitor, then that into the LED’s. I would use star grounding for the switcher IC and there are some critical traces. What frequency are you going to run?
The FETs must be as close to the inductor (=short trace runs) and the GND trace on the bottom FET is critical to return to the power input GND input node, also where the SGND needs to go. Otherwise noise will can make the switcher unstable. The LED’s are high current filtered DC so less critical layout.
Check out the PCB for the LM2743 eval board, it shows some of these concepts. I didn’t like the use of vias for the fet gnd. http://www.national.com/an/AN/AN-1356.pdf