Any thoughts on this? I have seen a handful of videos from some of the better youtube channels that show some pretty positive results from this bullet. I would be using it in my 4 inch K frame or my 6 inch Security Six.

38 Special 100 Grain Xtreme Defender | Underwood Ammo

My experiments, with .380 & .40 calibers have been promising. The wound tracks resemble the ones made by hollow point ammo. Not anything near manufacturers claims. To be fair, the manufacturer says that water-based gelatin is a better indication.

Q: "Could the performance of a new bullet design fall outside the accepted reliance on the permanent crush cavity in determining a pistol bullets effectiveness?"
DocGKR: No.
Lehigh Defense Xtreme Defender Terminal Performance

The clear-gel test video clearly shows the Hydra-shoks were better. They had a more appropriate penetration depth and expanded to a larger diameter. Do not measure the track width when evaluating pistol ammo.

Before asking "What ammo should I use?" you should ask "What should ammo do, what shouldn't it do, and how is it tested?" I recommend the following:
Handgun Wounding Factors and Effectiveness from the Firearms Training Unit of the FBI
Basic Wound Ballistic Terminal Performance Facts by Dr. Roberts (DocGKR)
Ordnance gelatin test assessment for rifle and pistol calibers by Dr. Roberts (DocGKR)
What's Wrong With The Wound Ballistics Literature And Why by Dr. Fackler

For specific recommendations I start here:
Service Caliber Handgun Duty and Self-Defense Ammo by Dr. Roberts (DocGKR)
Then here (for 9mm and 380):
9mm Ammo Quest on Youtube by ShootingTheBull410