Since scavenging and backpressure can have an affect on flow in & out of the combustion chamber, chances are that to get the most out of it, jetting may have to be adjusted to optimize the benefit. Because converter performance doesn't show up on the dyno, track testing would be the best.
i.e.: When I switched from open TTI headers with collectors to a 3" X-pipe system, which stopped at the rear axle with straight through DynoMax mufflers, my particular combo ran better with jetting 2 #'s leaner than before. I saw a slight performance boost which was easier to see because the extra 60 lbs added by the exhaust system was removed from elsewhere in the car. Comparisons were at the same weight with lots of logged runs over the previous years. (Your results may vary.)
