I believe it would be important to shoot a .45 ACP gun with the same weight as a .40 caliber gun and THEN compare the recoil. I have a .45 ACP 1911 that weighs ~40oz and it has pretty tame recoil. I have a ~30oz XD(M) .40 so it isn't a fair comparison off the bat.
I would like to hear from someone who has shot a 1911 in .40 and .45. Or someone who has shot a Glock in .40 and .45. That would be a much fairer comparison. Same weight, same bore axis, same everything but the caliber.
To make it even better. You need to compare the hottest loads in each caliber. i.e. Not a really wimpy .45 load vs a self-defense .40 load.
Plug in your chosen loads into this calculator: Recoil Calculator
Use the same weight for each gun. You will see that the .40 then has slightly less recoil than the .45 (and a higher capacity). I did notice that the recoil was pretty close to the .45. It doesn't split the difference between .45 and 9mm. It is pretty close to .45. And then putting it into a light polymer-frame gun doesn't really help in comparison to the all-steel 1911s many people are shooting their .45s out of (no offense G21 shooters... maybe you have a G17 and can comment?)