In wearing a kahr p40 so its not that heavy. However, the Velcro seems VERY strong. I'm sure it would work well for just about any cc type handgun. As for puttng the Velcro in your existing belts they recommend stitching it in. I don't think I'd want it stuck on with just adhesive.
Keep in mind when considering Velcro, its not like the gun is just hanging there with its entire weight supported by the Velcro. Youre IWB and belt tension has the holster pinched against your body. However, the Velcro is strong enough to dangle my kahr freely.
In the first pic, you can see the bottom of the velcro clip just under my belt. The clip can be adjusted properly (I was in a rush to take the pics, sorry) so that no clip shows beneath the belt at all.
In the next pic, you can see the view from the back, same story with the clip showing a bit under the belt because of my rush. I guess it's okay, cause you can see the location of the clips as I've got it set up:
In this pic, I pulled my t-shirt tight and shot from my view looking down so you can see how it is attached:
Then here, the same view with T-shirt tucked and bloused:
And last but not least, the Kahr P-40 with loaded magazine dangling freely from velcro, despite the one clip only hanging on by a corner due to the odd angle of drape lol:
I don't tuck my T-shirts in, and I think my outfit looks dumb with the big fat belt and tucked-in T. The point was to show how well these clips work with a good belt :D
Hope that helps.