The stock flap is adequate enough if you don't go through puddles. I have no complain for mine and yeah it's been my winter commuter even through the storms in the past month or so.
If needs must, it's wider and not longer flaps if you insist on riding through puddles.
I have the same problem on my commuter, if you’re riding quickly your feet are gonna get wet whatever bike/mudguards set up you have .