Sorry, not a mod in the usual sense, but as many are not happy with the factory mats, and not in love with the rubber-look Weathertech route, the Findlay mats are a nice alternative (though sure do not claim to be the first to use them). Thanks to whomever who suggested them originally.
They are incredibly good quality. What I also like is that they cover much more of the floor and have a ridge around its edges, so I don't end up with a ridge of dirt/junk outside the perimeter of the Subaru smallish, (and quickly-wearing) mats. For example, for the drivers corner, it covers the dead pedal, goes up much higher up the base of the firewall, and has a "wear pad"; similarly, it cover much more of the passenger's floor; and, in the back, the three-piece set covers from one side to the other, including nicely over the hump.
These are very washable, very high quality, and especially for U.S. Customer, the current exchange rate reduces their price nicely. Like their thickness (but it does not interfere with your driving).
F518 Style 3D Car FLoor Liner--Subaru Impreza WRX STI Waterproof All Weather 3D Car Floor Mats | Liners