By the time you fit a rear set seatpost to the smaller frame to position yourself back at KOPS there will be NO difference anymore.
Exactly, if you use KOPS and a constant distance center of the BB to the top of the saddle, then you're going to end up with the same "effective" seat tube angle on any bike that you set up this way. However, the actual seat tube angle will determine how easy it is to get a KOPS fit (if that's your goal).
Dru