The empty cans will create turbulence in the exhaust. Get the mid pipes custom made at a muffler shop - it's cheaper. I gutted a main cat on my Supra (N/A) and it felt worse than before. I even get some popping (explosion of unburned gas) in the empty can. If you want to open up your exhaust, the restrictions are as follows: 1)Pre-cats (gut them), 2)Mufflers (get cat-back system), 3)Main cats (get mid pipes - do it last)... My 2 cents.