I agree that the thermal relief may not be needed if this going to be reflowed; if it's hand-soldered it could be very difficult without relief. But I disagree about that 275 degrees; it is very misleading. The only way this device will work at 275 degrees is if it's dissipating zero power.
But, still, there's a lot you're not saying about this design. If this resistor is dissipating 1W, where is that heat going? Is it just building up inside your plastic box? Is your plastic going to melt (most plastics melt below 250 deg. C)? As FvM points out, there's a bunch of other stuff besides this resistor to consider.