So one character can choose for his "spells" to do ANY kind of energy damage?
Very powerful. Using WotC rules that would take something like 8 feats (Energy Substitution 7 times, plus the prerequisite) and the spell takes up a higher slot (1 level higher, I think). If the character can choose just ONE of those energy types, then that is more like the Energy Substitution feat and I think giving that feat to sorcerers for free would be a nice boost, but not crazy.
The Elemental Savant PrC has all energy spells converted to the character's chosen energy type automatically (without raising the spell level) but can't choose to do the original type. So my character Martan (in my sig) has all his energy spells do electrical damage, but can't choose to, for example, cast a regular Scorching Ray that does fire damage.