I've been pondering this a lot because I'm about to convert a 357 wheelgun to a true 9mm (custom cylinder and .355" barrel) in order to add magazine feeding.
For a personal defense role, the very best 9mm+P+ loads are very, very good. I think the Ranger-T 127gr projectile from Winchester and the Federal HST 147 are actually better slugs than any of the Speer Gold Dot. Not by a huge amount mind you - Buffalo Bore's 9mm+P+ Gold-Dot-based loads are quite good. But I suspect that the Federal and Winchester "Police Sales Only!" +P+ loads are now better.
The 357Mag still packs more energy of course but not by a huge amount. I don't think a lot is lost with the 9mm+P+ in a 357-strength (and weight) gun for personal defense. The 357Mag with the best loads still comes out on top despite the more "primitive" Gold Dots being the best slugs available. But not by much.
For hunting with heavy hardcasts? Ah, no, the 357Mag curb-stomps the 9mm-whatever and the 357Sig and gives the 10mm a run for it's money with the very best loads.
I'll say this though: if the best 9mm hot loads that we have today existed in 1986, we wouldn't have ever needed the 40S&W and I can make a damned good case that we don't really need the 40 now. A lot of people I respect are saying the same thing, including Tim Sheehan at Goshen Enterprises, Gabe Suarez and more.
Oh, and if you're curious about the high-cap mag-fed revolver project:
http://thefiringline.com/forums/showthread.php?t=489006
What else...ah. If you're looking for a stout 9mm+P+ gun, the ultimate is to start with a Glock in 40 and convert it to 9mm with a Lone Wolf or similar barrel, which is 9mm on the inside, 40 spec on the outside. Won't cost all THAT much, the gun will be double-caliber and it will live nearly forever with 9mm+P+ monsters.