The higher wattage PSU may be less efficient at your idle/low load levels than the lower wattage one.
An extreme example LZP-550 vs LZP-1000:
Power lost as heat vs load @ low load to 90W for the 550W is 7-8W, for the 1000W it's 14W
At 200W load, the LZP-550 has 13W of heat loss, the 1000W uses 15W
At 300W load, they are both around 21W.
Above 300W, the LZP-1050 is more efficient than the 550W.
Figure the LZP-850 is somewhere between those two.
e8400, Ninja + Thermalright bolt-thru kit, Gigabyte EP45-UD3P, HD5770 + AC L2 Pro, 4GB RAM, 128GB C300, WD640AAKS, Samsung DVD burner, Corsair HX520W, Solo case, low rpm Scythe fans, Fan Mate IIs. At the wall: 80W AC idle, 145W playing WoW, 195W playing Guild Wars 2, 215W playing Star Wars: The Old Republic and iTunes (near 100% load on CPU and GPU), 235W stress test (Prime95+Furmark)