Quit stealin my ideas. :ponder
Depending on how long the aluminum section is, you wont need one (minus nysinsp). the soft properties aluminum has actually acts like a muffler by itself. Aparently the sound waves are somewhat absorbed by the aluminum and it muffles the sound a bit.
Look up EVO aluminum exhaust on youtube. I saw a clip a while back when i was lookign into this too. It was SS DP into aluminum tubing back to the bumper, straight piped and it sounded like a normal cat back 3" SS did.
Adam is spot on, gotta find the temp where it will hold up. Also keep in mind, that SS has poor thermal conductivity, meaning it takes a while to pick up the heat, but will hold it in longer as a result. Since aluminum is the opposit, it will absorb the heat faster and also as the car is moving, the air passing over will wick it away quicker too. What I am getting at is say the SS DP is around 800F say 3 feet past the turbo (safe for heavyer aluminum) you might be able to get away with startting the aluminum around 2.5 or so or maybe less.
Or I am completely backwards and you will need to start it 4 feet away… LOL but I think you get the idea.