I just bought the PCS (?) support brace and rode six hours of rough desert singletrack with it on this past weekend (Desert Mountain Enduro course). This is the type that ties to your chest protector. I got the standard 1 1/2" version. The brace was virtually unnoticeable when sitting looking ahead or left or right. I noticed no restriction in my movement side to side. Standing up riding on generally level ground or uphill was also not a problem. However, when riding down hill and standing (which I generally always try to do) I definitely noticed it rising up into my helmet as I tried to lift my head to scan ahead. This would cause me to move my shoulders unnaturally in an attempt to reduce the contact with my helmet. The result was a very unnatural riding position that started to fatigue my neck more than normal. I then found myself sitting when I'd otherwise be standing (and riding faster).
Overall, I like the brace and think that, after a few more rides, I will have adapted to it well enough to forget it's even there. I'm going to try anyway as I can see the obvious benefit to this type of device. Based on the pictures I saw of the EVS, it looks substantially more restricting because it looks fatter. I would either shave it down or buy the 1 1/2" PCS unit. The guy at PCS talked me out of buying the full 2" version, and I'm glad as that would definitely have been a mistake.
I think it will be much more uncomfortable on the MX track where you spend much more time in the "attack" position (similar to the position I was in when I noticed the discomfort). Time will tell.