I will be swapping out my exhaust this Winter on my Challenger and I am going to use the TTI 2.5" X-pipe assembly (no mufflers) but buying separately the Magnaflow mufflers. I have been bugging the crapt out of the Magnaflow Tech guy asking him questions and trying to determine the same thing as you, how loud will it end up?

Something that might help you is that I asked the rep to tell me which Magnaflow muffler would be louder between this oval and this round style:
#11226: 2.5", 4x9x14 oval
#12616: 2.5", 6dia x 14 round

His reply: ""They have almost the same internal volume, so they would sound the same."

That to me makes sense. So I am going to start with the 4" x 9" x 14" ovals under the seat with the stock TTi 2.5" exhaust running out the rear. I will start it up and see what it sounds like. My guess is that it will be fairly loud so then I will be looking at adding a pair of the 4" x 14" long round mufflers beside the gas tank to see if that helps out on the noise. After reading some threads of other members, I might need to use two mufflers up front and two out back to keep it where I want it. I think that it will be trial and error.

If you need the Magnaflow Techs email, pm me and I will get that to you.