^^ Felt like less than an 18mm difference. Since I was looking to shorten the overall reach anyway, I used the same stem.
Now I am considering a steeper -17 or -20 angle to get a larger drop. (I feel I could go lower than the shallow drop bar provides)
BTW any angle would reduce the actual stem length. (though one mfr ??? measures length based on effective length and accounts for the angle in their measurement)