There's nothing wrong with the sentence. :) "If you were to write out your brain's potential..." - How? In what way? Creatively or technically? Or another way? "...as expressed by the number of possible neural connections it could make..." - In this way. So, we have a quantitative measure. Removing "as" would be a mistake. The sentence would become broken. Adding "it was" would also be a mistake. The useful phrase here is "as expressed by (this)", to mean "shown in this way".

I think it would be okay to replace 'as expressed' with 'expressed'. There is nothing omitted between as and expressed. I hope I helped you out. Do you have any other questions?