What does 所 mean in 正如我们之前所说?
正如我们之前(说)= Just as we (said) previously- 说 is a verb here

正如我们之前(所说)= Just as (the words) we (said ) previously

"我们所说" is short for "我们(所)说的話" = "the words (that) we said" 

therefore, "我们所说(的話)" is a noun

When you say "正如我们之前(说)" you have to follow it with a quote of what was said

When you say "正如我们之前(所说)" you can follow it with a quote of what was said or omit it and presume it is known