For me, it isn't the issue of which spells can be added in, or whether the pact will be enforced or not, although these are considerations.
For me, it is "+freaking 4"!!!!
You should never balance out +4 feats with roleplaying elements. You should balance it out with -2 something else (and even then, +4 feats should almost never be possible).
+4 is just too huge.
For example, 14th level sorcerer one has a DC 23 save with Finger of Death. 14th level sorcerer two has a DC 27 save with Finger of Death because of this feat.
Opponent one has a +7 for Fort saves (which is not that bad for many 14th level characters, the same level as the Sorcerer). Against sorcerer one, he saves 25% of the time. Against sorcerer two, he saves 5% of the time. It's not just a matter of 20% difference, it is a matter of 5 times difference.
25% is a low chance of success, but 5% is practically non-existent.
And if you cast this spell against a same level opponent with a very good save (e.g. +13 at 14th level), the odds of dying move from 45% up to 65%. That's extremely significant. The Sorcerers CHA would have to increase by 8 with respect to these same spells and he would have to be somewhat demi-godlike to get this same effect (i.e. it would take 32 levels of increasing CHA every four levels to get this same level of effect).
Save or die type saves are hard enough to make due to level, caster ability score, ability score items that boost abilities, and feats like Spell Focus without additional feats that boost them by +4.
This feat is Broke with a capital B.