Very secure. In addition to what has been said, you don't have to worry about the spring bar on the buckle popping either. The buckle is very well sewn into the strap.
I have used this method when adding the Countycomm strap adapters. It works very well.I think someone made 1mm spacers from a plastic ball point pen tube. One on each side of the strap should do the trick. :-!