Hello, sir. Yes, I believe that "9mm is enough" with rounds that make the most of its ballistic capabilities. I have personally seen the effects of both 9mm and .45 ACP against humans and both can "work" splendidly...and not so well at other times.
We ask a lot of our handguns and seek the elusive "one shot stop", which probably doesn't exist for physiological reasons unless we get a solid central nervous system hit.
Speaking only for myself, "placement is power." In other words, if you have a cartridge of adequate power for expansion and sufficient penetration, your ability to shoot that cartridge/gun combination outweighs just caliber/load characteristics. If you can get the hit or hits with the 9mm, go for it and I think you are about as well protected as you can be with any handgun caliber normally associated with self-protection. If you do just as well with a .45 ACP, then the decision boils down to which you prefer, magazine capacity or heavier caliber, Glock 21's, XD's, and some others being the exception as they hold lots of forty-five rounds.
I use both 9mm and .45 ACP on ocassions when I tote a "serious" pistol. I believe that the .45 ACP with the best loads probably is more potent than the 9mm with similar loads, but I do not believe that the difference is quite so great as some other folks might. It is not enough that I'd carry a forty-five I couldn't hit with in lieu of a 9mm that I could.
Most of us would probably opt for some sort of long gun if we knew a deadly force confrontation was coming and:
1. We couldn't have a SWAT team to intercede, or
2. We couldn't escape and avoid the whole thing.
The problem is that we likely won't wake up knowing that today's the day I get into a shooting situation and we won't have an FAL, AR15, or shotgun at hand. We will probably have our defensive handgun, which is underpowered compared to about any 12 ga. shotgun load or rifle round at typical defense ranges.
Thus, it seems to me that we want:
1. A reliable handgun,
2. In adequate caliber that we can handle,
3. Loaded with expanding ammunition,
4. Coupled with the actual willingness to shoot if no other reasonable alternative exists and the skill to do it effectively.
Much debate exists over what is "adequate" and "best" loads for these calibers, but to me it seems that even more important are reliability, skill, and the proper mindset.
I'm about as happy with a 9mm Hi Power loaded with high-performance ammunition as anything. This one is usually loaded with either Corbon's 115-gr. DPX or Winchester 127-gr. +P+ Ranger RA9TA ammunition.
Today there are a number of rounds that seem to "work" well. From left to right: Federal 115-gr. JHP, Corbon 115-gr. DPX +P, Hornady 125-gr. XTP (TAP), Winchester 127-gr. +P+, Remington 147-gr. Golden Saber, and Speer 147-gr. Gold Dot.
Corbon 115-gr. DPX +P seems to expand reliably in about any "soft" target it hits and does so with or without intermediate barriers.