Hi!~ I also have a Montna Van, 2003.I ahve done the exact exhaust mods as you, but I also added a second cheater pi'pe. and have the duals out the rear. I also added spoilers, low profile tires and rims. Running boards, a Venom 400 box and a Triphase box as well which both add noticeable power. Looks GREAT! In fact, if you go to www.cardomain.,com and type in rpalais in the search bar, it will take you to my garage page and you'll see the 2 Pontiacs I own as well as all of my other cars I collect.
I also buolt a 1979 LeMans Safari and have a ground effects kit on it as well as a '67 GTO 400cid with Holley EFI and a 4-speed AOD from a Corvette. it screams!
I too, am looking for more mods for my Montana as it is a very heavy Van which slows it down, especially with the AC on......Please let me know if you ever find out any other mods. I am toying with putting on a Supercharger like the one they use oin the Grand Ams, etc....It is a Vportech style that has a long shaft with a pulley on the end.