They are Emgo reverse cones, and I was surprised how good they sound (much lower and deeper than my phone could pick up in the video). I hope that it runs that smooth throughout the rev range. I got it up to about 4,000rpm and it did fine, but I was afraid that it was going to walk off the centerstand, so I didn't rev it any higher.
I'm interested in those Engo reverse cones - what length did you buy? MikeT307's link above refers to the 17" model, just want to make sure that's the right length before I order 2 of them.