Are peltier devices linear?
No, because power is a product of the voltage and current:
15.2V * 6A = 91.2W
5V * 2A = 10W

You would need approximately 9V @ 3.6A, which would give you ~32W

Remember, peltier devices are not very efficient, so 30W of input will not give you 30W of cooling power.

Wikipedia cites Peltier devices at an efficiency of approximately 5-10%, which means that for a best-case application cooling a 10W heat load, you will need a 100W peltier with a heatsink that can handle the aggregate power, or 110W of dissipation to maintain the 10W heat load at ambient temperature (this is assuming a 0° C/W heatsink on the hot side of the peltier).