Mulling this over: words can, in a sense, be material--we can record them on paper or other tangible things. Letters are symbols but they have narrowly defined meanings. Letters combined can become words that represent material things.
Performativity is, by its very nature, ephemeral, even if it is in some sense material (bodily, as discussed above). The premise, then, seems to be that if discursive practices are performative, they naturally challenge power because power can only be obtained upon something longer-lasting. Or, alternatively, that performativity challenges power because the understood meaning(s) cannot be represented by words.
Doesn't the assumption that words have power act to give words more power?