the ballistics given by winchester put it's 147 GR. RA9T MV at 990 f/s, it's mass is 147 grain which results in ~433.65 Joules of KE
it's 124 GR +P cousin (RRA9TA) gives a MV of 125 f/s, it's mass is 124 grain which results in ~583.18 Joules of KE
Well that's impressive, but they had to add MV to overcome the lower mass, what about a standard pressure 124 grain round? Well winchester doesn't have one in Ranger-T but We'll look at a comparable JHP round, like Federal HST 124gr JHP (P9HST1).
the stated MV is 1150 f/s, it's mass is 124 grain which results in ~493.61 Joules of KE. clearly still more energy that the 147 grain RA9T
So now lets examine the law of kinetic energy. Basically, the amount of work needed to stop an object in motion is equal to the amount of work needed to put the object into motion. The standard pressure rounds likely use the same amount of work to put them in motion so we can surmise that it will take the same amount of work to stop them, but the +P will take more work to stop them.
Please keep in mind that there are more complicated factors involved because these objects are rotating about a center of mass, and that makes calculation much more difficult to calculate. So much so, that it is beyond my mathematical skill to do.
My conclusion is that the +P round could result in over penetration unless the target was covered in many thick, dense layers of clothing. however in ballistics tests where they were fired at many layers of denim, they all exhibit adequate penetration into ballistics gel. Ballistics gel does not account for bone tissue, so considering that we are trained to shoot at center mass, take that for what it's worth.
I don't have any chambers for 9mm, but my girlfriend does. Because every 9mm ballistics gel test shows no issue penetrating several layers of denim and still making its way well into a block of ballistics gel, I see no real advantage to +P or +P+. I only see risk of over penetration and/or increased wear and tear on the firearm. The only instance I see the use of +P+ ammunition to be worthwhile is when you are shooting into an angry mob, where over penetration may be an asset rather than a liability. For practical self defense situations, I believe a good quality 147 grain hollow point slug is adequate to get the job done! For my money it's gonna be Winchester Ranger-T series 147 GN, standard pressure.