What kind of aluminum mufflers are you using? My car slows down about a tenth and 1 mph w/ my "full" 4" H pipe exhaust system on the car, but I bet it's more due to the weight than a restriction issue. I've never weighed it, but it's a lot. Most of the ET loss is early in the run.

CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax