A regular foam collar will provide nearly no protection against a compression injury. Neither will it help with hyperextension, hyperflexion or subluxation of the cervical spine. It compresses under force, allowing impact force and momentum to do their damage nearly unhindered.
I don’t know what you mean by “free” fit. Do you mean greater range of motion?
Yes there were some product testimonials to the Leatt on the old site. You can still look those up if you look for the link at the bottom of the new EKN home page.
You don’t need the collar to be fixed to the helmet, but if you’re worried about the collar not being fixed to the body, most of the collars you mentioned do come with optional tethers that fasten around the ribcage. They are not required, however, for these collars to do their job. Instead of the helmet being restrained from the back by tethers, the motion of the helmet is restricted by the shape of the collar, and then is transferred to the body and made less traumatic by spreading out the force of the impact or abrupt change of direction over the greater surface area of the base of the collar. In the event that the head is thrown towards the rear of the body, tethers at the back will do you no good. A HANS device doesn’t have to be concerned about that, as cars have headrests to prevent hyperextension.
As for concrete evidence, the one way that anyone can be sure is if you do an A vs. B (new collar vs foam) to the same person and under the same circumstances. I, for one, am not volunteering. The other would be for an independent body, such as Snell, to do testing. No organization has yet stepped forward to do so. So you’re going to have to go on what anecdotes you find, and the data of the manufacturers, that is, if they’ve actually tested their product.
I use a Leatt. I do so partly because they actually have done impressive testing on their collar. To my knowledge, the look-a-likes (if you go to motorcycle websites, you’ll see a lot of braces that look like Leatts, but aren’t) have not, and neither have EVS or Alpinestars done so on their models. Also, having held and examined Leatt, the new EVS and the Alpinestars collars, the Leatt is more substantial. There is a lot more plastic and flex in those, and so I have a difficult time believing in their ability to be effective. Also, the EVS and Astars collars clasp at the front, which can be the major loading point for force. That seems like a pretty bad idea. The Leatt is definitely more expensive, but I believe it to be the better product.
Am a trauma expert? No, but I am a nurse, and I would like to think that my knowledge and experience counts for something.
As for the efficacy of the foam collars: though I haven’t read the report and so don’t know the methods of their study, the FIA CIK deemed that foam collars created no discernible protection, and may actually be more of a danger to due lack of range of motion causing a lack of visibility. They recommend to not use the foam collar. They have not yet done any testing on the collars coming from the motoX world.
Ultimately, you have to make your own decision. It’s literally your neck after all.