Depends on the gun. M150 in either my M4 or CQBR PTW would shoot consistantly 455-460 fps with 0.25. As soon as I put the PDI barrel in the M16 (509mm), same cylinder shoots 480-490. Same BBs. All barrels are 6.04mm, but PDI is slightly under and Systema slightly over, maybe to the tune of 0.01mm difference between them.
Extra length does have it's advantages, regardless what she tells you.