Full exhausts to open headers gain

It's been 10-15 yeas ago, but after several years of tuning my 360 Dart Sport with TTI stepped headers and long collectors, I switched to a 3" X-pipe ending with Dynomax Ultraflos at the rear housing. It picked up a bit, then a bit more after I jetted down 2 #'s. The weight of the exhaust was removed from elsewhere in the car to keep that factor consistent. I don't think it was over a tenth, but for class racing it was certainly worth keeping it on and my ears liked it too! (Mid-low 11's up to 118 mph @ 3,550+ lbs.)