I'm sure the mods will move the thread, but to answer the question in the meantime...
There really aren't any bolt on modifications that will make much of a difference. You can get a chip and a smaller/lighter underdrive pulley, but the impact will be negligible, and you already have the better top end, so you don't even need to upgrade to that.
In order to REALLY unleash the LQ1's performance potential, you need to do some custom headwork, and slap on a custom turbo set-up. The combo of these two will make the supercharged 3800 feel like a Chevy Metro. It is not uncommon to put down well over 200hp at the wheels with just headwork, or 350 at the wheel on a turbo set up with low boost. Unfortunately, none of the parts for this will be off the shelf stuff, or cheap. That's the main downfall to the LQ1.
For the exhaust, I've never really heard ANY mufflers sound bad on the GM 60° motors, as opposed to the 90° motors where I haven't heard ANY exhaust set up sound GOOD.