Don't count on your bullets going that much faster just because you have a longer barrel. I have two FA's in 454,one is 6" the other is 10" at most i might get the 10" to run 50fps faster than the 6" with the same load.

I believe the cylinder gap is more critical than barrel length when it comes to velocity. And some guns just shoot faster for what ever reason.

As far as a barrel length, i don't think there's any magical number. My preference is like what most of guys are saying,6 to 10" with 7 1/2 probably being the most common.