Try setting down the texture quality from the game settings. It should cause Warcraft III to skip loading the highest mipmap level of large textures. I would imagine that having massive memory savings for such asset spamming maps. The textures will also not be any worse than the standard Warcraft III ones.
Outside of style consistencies, there is no reason to update the icons. Warcraft III icons are 64x64 pixels. Even on a 1920x1080 display they are like 69 or 70 pixels odd. Since the icons do not use mipmaps, using a 128x128 icon would be counter productive due to aliasing artefacts on most display resolutions, similar to why HD textures for terrain is counter productive.