So i guess it's impossible.
WiiU uses a special file-format. And it might be that's why 300 = max. It means ~300 mbytes of picture-data (each picture 1 mbyte). Since it takes 150 seconds = 2 minutes,30 seconds to load those ~300 game-icons, there's your problem. It might be, this is the max. waiting-time coded into the Os to load game-icons...