Not to nitpick or anything (OK, I'm nitpicking), but even with Deathblow you still need to fulfill the other conditions of a coup-de-grace (i.e. the opponent being helpless). Lacking Dex bonus is miles away from helpless.
I've found most rapier builds, keen or otherwise, not to be terribly effective unless the character has lots of power behind his blows. Most of the time, it's just another +1d6 to damage.
