I thought I read, a long time ago, that there's a cap to how much you can temper the Ghostblade. If so, anyone know the cap? Thanks!

Man, Ansilvund is a tough dungeon. Just did it on Legendary with a fighter/dragonborn orc. Not using followers, conjuration, scrolls, or a ranged attack. Slow Time was the best Shout for this fight though it took about a dozen tries to defeat Fjori and Holgeir. I didn't want to kill Lu'ah Al-Skaven first to end the fight quickly, wanted to really take on her 'servants' and beat them in melee.

Doesn't have a cap, like all other items, you can smith it forever... Of course only if you have a large supply of Ectoplasm...

Or you're prepared to wait a long time before using the three you have!

The other difference from other weapons is the lack of associated perk. You don't get any doubling effect even at 100 Smithing, so it improves less rapidly.