The strongest katana is +4 but requires an upgrade from the imp in ToB.
There are only a few enemies that are immune to +3 weapons though. Demiliches are among them, but the last boss of ToB is only immune to +2 or less, which is the exact same as the last boss of Siege of Dragonspear…when you are only level fucking 11 which is wild. It was easily the hardest boss in the series.