That's a custom-made intake plenum, boys and girls!
I'm pretty sure it was made by the same guy that built his engine.
Here's some more info I found out tonight:
Dennis bought one of the tubular-frame Miatas from Racing Miata and is putting the engine featured above in said car! He said it's just too much for the street and wants to build a track car. He will have another engine built for his yellow Miata that's a bit more streetable. Yeah, like 400hp maybe, huh? hehe
ALSO, Dennis hasn't given out specific numbers on a 1/4 mile for his Miata, but he said it was faster than his Porsche, which is the car in the link above that pulled a 9.90!!