John, you will find LOTS of opinions on this topic. Even in the MX world (I did that as well) you will find tons of differing opinions, even on the Leatt itself in spite of the testing they have done and shown. No easy answer here. Body shape, seat position, kart, etc. all affect how they fit as well which would affect performance.
We use the foam just for help with fatigue of the neck. My son used the Leatt in MX without fail or problem, but we can’t get him comfortable with it in a kart. Have looked at the others, but PERSONALLY, would prefer the Leatt or are looking at the Vahalla, which seems to have the best coverage as compared to the others.
That being said, all experience and advice here is anecdotal. Without testing, hard to say what works. Even though Leatt has done lots of testing, its more on the MX side.
I had also heard that HANS was making a karting one in an article in one of the magazines, but haven’t seen it come to fruition.