I think a better formulation would be "I exist, therefore, I feel." Existence comes before feeling.
I tend to agree with Jefferson, but I am not ready to disagree with your statement either. Given my understanding that Epicurus is taking the information provided by the senses (and prolepsis and feelings) as the ultimate way we determine what is true, I could see an argument that Jefferson's sequence is correct, and that we start with the senses/anticipations/feelings as given - because we have to - and go from there. I could also see some kind of combination that requires both simultaneously, and I tend to think that the decision of the Epicureans to combine canonics and physics may be related to that.
I hope we can find some material at some point from people like Sedley or others who have spent more time with what Sextus Empiricus has to say about Epicurus (which i have not studied). And surely there are many critical commentaries on that "I think therefore I am" formulation. Until we look at it further I better reserve taking a firm position on that one.