Skipping over whether you should avoid trashing a Ultegra drive train by exposing it to European salted winter roads daily, and get a winter beater bike instead ...
Sintered pads will perform better in the wet that organic ones.
The biggest downside to organic brake pads is that they do not perform well in wet conditions. In wet and muddy conditions the brake pads can wear down even faster than normal and could also get glazed over keeping them from performing again in dry conditions.
Sintered brake pads will continue to grab as strong as ever in wet and muddy trail conditions.
The downside is noise and higher wear on the rotors.
The few negatives to running sintered brake pads is that they can make some noise. Depending if they are wet or have been really hot, sintered brake pads can be loud! Lastly, because sintered pads are made from a harder material, they can be harder on rotors. For most riders, this usually isn't a problem because it takes some serious abuse to burn up brake rotors.