Wasn't this the idea behind the Ruger Compact Magnum line of cartridges though? I would agree more barrel = more time for the gas to accelerate the bullet.The load used to give you the most velocity in a 24" barrel, will also give you the most velocity in a 20" barrel.
That a Model 700 XCR Compact Tactical? Remington doesn't show an SPS Tactical. Either way the rifling twist is 1 in 12 for an XCR. The SPS has 1 in 10. Same rifle with a different finish and barrel anyway.
"...That load is over book max..." Nope. Close to max(45.5 compressed for a 165. 45.4 compressed for a 168 with IMR4895 are max loads.), but not over. What book do you have?