I'm really not a "fragmentation guru", but I did stay at a Holiday Inn Express last night... So I guess I'll chime in here since no one else has.
The Short Answer:
Commercially loaded 223rem 55gr FMJ ammunition may or may not have the same fragmenting properties as M193 Ball ammo.
The Long Answer:
The market for budget grade commercial ammo (UMC, USA, American Eagle, etc.) is very competitive, and if they find a way to reduce manufacturing costs it's a pretty good bet that they'll jump on it. Consideration of how it will affect the fragmentation of the round is the furthest thing from their mind when making cheap ammo. If it happens to perform the same way as military ammo, it will be entirely by chance. They figure that if you're actually planning on shooting something with a pulse, you'll use their premium ammo, where terminal performance is a major factor influencing cartridge design.
You can do your own gelatin testing, but the bullets you test today may not be the same bullets that they are using when you buy the ammo again in a few months. For example: over the last 10 years I have seen Remington make at least 3 changes in the design of their 55gr FMJ 223 projectiles. And before you sarcastic folks ask: no, I don't run out with my calipers and compare every bullet in every box of ammo that I buy. Even though I may not have much of a life, I actually do have better things to do with my time. The changes I'm talking about were subtle, but if you were paying attention they could easily be seen with the naked eye.
Additionally, as you mention in your post, the muzzle velocity of "generic" commercial ammo is often fairly low. Less gunpowder = lower production cost. All other factors being equal, this factor alone could easily affect the fragmentation. On a couple of occasions I have seen American Eagle 223 ammo that was so underpowered it wouldn't even cycle the action. Now granted, I may be splitting hairs here (I wouldn't like to be shot with ANY ammo, either underpowered or high powered), but I like to have as many advantages in my favor as possible, so if I'm the one pulling the trigger I prefer to use ammo with some "oomph" to it.
I hope this rambling response answers some of your questions.