I wouldn't expect to see significant lap time changes between two cars equipped with competing intake/header/exhaust combos. At best you'd be seeing gains in the 5-6% range (average) over the usual operating band. On a track where lap times are in the 60-90 second range, that might net you a quarter second, half second max on a track with long straights. And that's assuming a driver capable of consistent laps (as richard said) and two cars with identical suspension/tire setups. The only way that'll happen is if you swap all the parts on the same car. Even straight speeds don't tell you everything since it depends on corner exit speed as well.

Nope, dyno and instrumented acceleration testing is probably the ideal way to measure the power gains.

UL