Define 'lag'... What you are talking about is comparing full-throttle 4th gear dyno performance. Sure, with 2.5" piping you may see a low-end improvement on your dyno chart, but smaller piping is still going to be more responsive for part-throttle transients on the street. Most dyno runs consist of mashing the pedal at ~2500rpm to measure peak output; very little, if any, attention is paid to the actual throttle response. As mentioned above, it's a balancing act.
