Thanks to Makoman for this video:

Here's the +P video paired to the above:

Basically, the argument is that technology has improved ammo to the point where benefits from +P are negligible. He mentions +P gets a little more expansion while standard pressure gets a little more penetration, but overall performance is about the same with modern technology loadings...

Which for handguns, I largely agree with considering the recoil, flash, and bad habits both of those encourage are all overall negatives to accuracy -- +P, +P+ 9mm in a lot of ways has more in common w/ 40 s+w in how the gun handles...

Some bullets like Hornady critical duty and older technology like the federal 9bp/bple bullets do perform better at higher velocities and benefit from +P, +P+ velocities.

Out of carbine/pdw with a longer barrel, generally you don't feel increased recoil, the +P/+P+ gives you even more velocity gains than you see out of pistol length barrels, you shoot flatter, and gives you an extra 50 yards in so in velocity vs standard pressure loads.

Keltec includes this graph in their sub2k owner manual:

And here are some #s I pulled together regarding 9mm vs 357 sig that I pulled together from ballisticsbytheinch for a 9mm AR or Keltec carbine:

All 16" barrel specs (and bear in mind 9mm are +P values, but +P+ is really the truest comparison).

357 sig Cor Bon 125 gr. JHP = 1723 fps

9mm Cor Bon 125 gr. JHP +P = 1430 fps

1723/1430 = 1.205, or 357 Sig is 20% faster than +P 9mm at 125 grains

357 sig Cor Bon 115 gr.JHP = 1768 fps

9mm Cor Bon 115 gr. JHP +P = 1525 fps

1768/1525 = 1.159, or 357 Sig is 16% faster than +P 9mm at 115 grains

And Gunblast has some +P+ 16" specs for the CMMG 9mm AR:

115 grains +P+ 9mm buffalo bore = 1656 fps

1768/1656 = 1.068, or 357 Sig is 7% faster than +P+ 9mm at 115 grains

http://www.gunblast.com/CMMG-MK9.htm?Shorter barrels like the EVO 357 Sig would have a little more advantage than longer ones though...