We build a lot of 2 barrel circle track engines and going from a 5.7 to a 6.250 on a SBC we see both make peak power at about the same RPM about 6800 but the long rod engine at 750o PRM will make average 30 more horse over the short rod engine.
In turn we can run more gear and turn the engine more.
Most tests are done with the same cam but the engines breath different between the 2 and do require a different cam.
Tom at http://www.3vperformance.com/
use to be my old business partner years ago and he has worked for some of the top cup teams Roush, DEI and Toyota and his engines that run down south are top engines in their classes and those are all 6.250 rods in those engines.
On our SBC blower engines we run 6.300 rods in those engines 8 years in a row 1st and second.
Some of the classes we build for have a 5.7 rod lenth rule I wonder why.