2 1/2" vs 3" Exhaust Comparison Video

are you guys convinced these tests are legit?
The last one showed a 335 hp 383 with a cam,headers and intake change increasing the output by 145 hp?
..and the stock 340 test with XE268 cam,headers and intake change putting out 391hp?
One day, I'm going to retire and just do dyno testing. Just because.