Negligable horsepower advantage? Measured at the rear wheel a good 250 done by PC or Eric Gorr will put out around 48 horsepower. A 310 done right will put out close to 60 horsepower. That's 20% more buttmunch, just enough to get a holeshot. I am serious as a heart attack.
When was the last time you heard the AMA tearing down an engine or measuring frame geometry?. A friend at Wiseco confirms my claim. They sell a of big pistons to pro teams. Wake up and wipe the boogers out of your eye.