Posey isn't only tearing it up in AAA he's also in a small sample size tearing it up in AAAA. I made the move for him today and did exactly what you're contemplating, i dropped Soto for him.
i'm in a 10 team league, so once you get outside the 7-8 C that all teams on the picking is slim but not terrible. Here's my rationale on dropping Soto (though he did go deep today).
1) Soto hits at the back of below average CHC lineup, but then again Posey will likely hit 6-7 in a nothing to write home about SF lineup
2) Soto has been struggling lately, Posey has been hitting better and better in AAA and now proving it in AAAA
3) if Posey busts, and Soto is scooped up, I still have Carlos Santana soon to be called up