I'm not making quite 500 hp but I did find that installing 3" x 2 1/2" reducers on my headers may have increased hp just a little. Take a look at my numbers from two weeks apart. The weather as about the same. Hot!
With 2 1/2 reducers
60' = 1.446
1/8 = 6.650
1/8 mph = 102.43
1/4 = 10.516
1/4 mph = 126.27
w/3" turndown
60' = 1.472
1/8 = 6.649
1/8 mph = 102.59
1/4 = 10.523
1/4 mph = 125.99
Notice that even though I had a better 60' with the 3 x 2 1/2 reducers, my 1/8 mile time was not as good. But I pulled more hp on the top end and passed that time up.
My 3 x 2 1/2 reducters have a 2.375 ID. I wonder what a 2.500 ID would do.
You can veiw my web page and see all my mods.
------------------
88 347, Twisted wedge heads,Comp Cams 224/230 cam, 1/4=10.516 @ 126.27, 60' = 1.446
http://xs-fx.com/raughammer/larrysample.htm