I won't buy them just because they call them 3" DP's. Not big on liars. More importantly, I don't see how going from 2.5 - 3 - 2.5 can increase flow or decrease back-pressure. Actually to me it only seems worse to expand and compress the air again. They should just be 2.5" all the way for the best possible flow.
If this were N/A car then the 3" center pocket can be useful for exhaust scavenging, but this is a turbo application which makes that not applicable.