Interesting setup, but I really doubt your human observation which is highly inaccurate. You need to have a speed gun to record the pace of both shots and go from there.
Assuming two shots have the same pace, one with topspin is carrying an additional force, ie spin, and your racket will need as much force to react to it.
Again, although they look the same to your eyes, I really doubt if a recreational player with the open stance could hit the same force as one with more neutral stance. Open stance gets power from leg and rotation which means a very strong core. In addition open stance produces angular impact which means less force than direct/linear impact of the neutral stance. My 2 cents.