Here's my long answer and opinion....
A few years ago I shot at a charity LE shooting match, they used IDPA style targets and scoring. I shot the S&W 9mm 1911 Pro Series pistol that I carried on duty and did well, coming on 6th out of about 50 shooters. The next year I went back with the same gun expecting to do better, except it turned out that the course of fire was almost all steel targets instead of cardboard. We were scored on time and hits and you were penalized for knockdown targets that were left standing. On a couple of stages they used big popper style knockdown targets and, much to the dismay of those of us shooting 9mms, the lighter calibers just didn't get the job done as well as the heavier ones. Where it would take 3-5 rounds to knock a target down with a 9mm the guys shooting .45s were dropping them in 1-2 shots. The guys shooting .40s didn't do much better than the 9mm shooters. I was slowed dramatically by the extra shots it took to get through the stages with knockdown targets and ended up placing 23rd, midpack. All of the top 15 or so shooters were shooting .45s with 230gr ammo.
Now this was not a test and there is nothing scientific about the results, no controls to account for individual shooting ability, etc., it's just my observations. I can tell you that it really bothered me that I wasn't able to do the job on the knockdown targets with my 9mm duty gun when other shooters of similar skill were taking them down with 1 or 2 shots using their .45s. I had about a 6 hour drive back from the event and thought about it the whole way and by the time I got home I decided to switch to a .45 for as much of my carry needs as possible, both on and off duty. I know that may sound silly to some of you but I had lost confidence in the 9mms ability to do the job. I ditched the S&W 9mm 1911 for a Springfield Loaded .45 and a while later bought the FNP45 Tactical that I've been carrying for the last couple of years.
SO, my answer to the OPs question is that, 1-2 shots from a .45 is equal to 3-5 from a 9mm. Again, nothing scientific to back it up, just my observations of the difference in impact energy between the two calibers. The heavier .45 hit harder and knocked targets down easier than the 9mm.