Current switching power design is around 80 - 90% efficient, and it doesn't eat full power all the time. I'll try not to get too technical, but essentially, once it charges up the power supply capacitors after turn on, it loafs along until your system demands power. Then, the caps deliver the initial jolt, and the power supply delivers what is demanded, filtered by the output choke and caps.
In other words, it probably won't dissipate much more power when idling than a supply with a substantially lower rating. If you don't need it right away, you can wait for a free after rebate hot deal, but if I needed the supply for the system you describe, now, I'd go for this one.