Like everyone said there is no reason you can't run a Plog/downpipe setup. You will just need a few more supporting mods I'd assume to run a 3.4 safely.

I completely understand why you don't wanna do headers. I think most people that get a downpipe kind of kick themselves in the ass for getting one because eventually they want headers.. That's kind of out of the question for me for various reasons, so I'm going the Plog/DP route, however I'm also N/A. good juck!