I agree and disagree with the comments made about the Mental Toughness/Improved Mental Toughness feats.
I took one of the Mental Toughness Feats as a Drow Sorc and I like it.
But then I did not take Finger of Death for my level 7 spell. I took Otto's Dancing Sphere.
I also am in a guild with a lot of melee characters. So I like using the extra sp to help haste/displacement/resist them when necessary.
When the level cap goes up, I may drop MT for SF: Necromancy (possibly); I will pick up Finger of Death at 15th level.
If you are using Finger of Death a lot (or any other particular spell) then the benefits of Spell Focus probably do outweigh the benefits of Mental Toughness.
That's my 2 cents anyways.