Everything is a compromise. Adding the siping and other tread block changes likely affect the wet weather performance in exchange for cold/snow/ice.
Summer tires need to be able to evacuate standing water from heavy thunderstorms and other summertime wet weather events... as such they are usually pretty good in the wet. Things change dramatically though once the temperatures get below 50 degrees and fall off a cliff below 43ish.
Good test video showing the difference in the wet: