I think even if Brett himself believes he can bowl one ball at 100mph, he will not do so as with his age it is very easy for him to damage a muscle or his back. However, if there was a match to be won and only the last bats man standing between victory and a draw/defeta and Brett knew he can get that last wicket with a 100mph delivery I am very sure he will go for it.
Even at 90-95mph he is good on his day if he gets his line right. Its not the speed which makes him unplayable but that he tries to out think the bats man.