I don't really have anything to say about katanas, so I apologise if anyone feels I should have posted this somewhere else.
I do, however, want to voice an item upgrade possibility.
If anyone here has the Valen Mod, and installed the Anti-Paladin portion, you know you can get the Unholy Reaver from the Gith Anti-Paladin. I believe you can encounter one if you carry around the silver blade and refuse to give it up, and I am fairly sure you can find one in Watcher's Keep on Carston's level.
Anyway, my suggestion would be to have Cespenar be able to merge the Silver Sword and the Unholy Reaver, with some kind of appropriately evil component and an appropriate sum of money. As the Unholy Reaver is approximately on the same level as the Lilacor +5 (both 1d12+5, Lilacor grants immunity to charm, confusion, and intelligence drain; Unholy Reaver grants protection charm and psionics), perhaps the percentage chance of a vorpal hit could be scaled down and the penalty versus death be removed, leaving it something like this: 10% chance each hit target must make a save versus death or die. I had originally thought the ability of the Axe of the Unyielding might help, but the axe of the unyielding doesn't require a save versus death.
I don't know how much coding or anything would be involved, or if it's possible. It doesn't seem too unbalancing to me, because the higher end creatures have excellent saving throw values against death.
Maybe someone could offer their thoughts? If there is any merit or inherent flaws in my suggestion? If it's even worth talking about?