I think you get higher losses at 10A - in the switcher, wiring etc. I would series connect more LED's to up the voltage, or break them up so you can use smaller 2-3A switchers.
Doc, I tried a few of the LM2596 switchers from eBay, I was going to use one for an iPad charger. They just don't have the heatsinking or diode rating to dish out 5V@2A from 12V, so for $1.59 they let me down. They run quite hot.
DX has some "10A" ones but they are potted: http://dx.com/p/dc-12v-24v-to-dc-5v-car-negative-booster-power-converter-silver-149111?item=14
A cheap buck converter will fail and overvoltage your load, so I don't cheap out on them anymore.