Depending on fixture design, the fluorescent may not achieve full brightness in the cold. And as the lamp ages it may not even start in extreme cold.
My recommendation is for a 100 watt high pressure sodium dusk to dawn fixture. It'll throw more light than your old 175 watt merc. And sodium is totally unaffected by the cold.
Or if you don't need continuous light go with a motion detector + 300 watt halogen flood.