I'd suggest going over your cooling system to make sure it's in good shape, with a fresh, 50/50 mix of HOAT coolant and distilled water, maybe swap in a 190º thermostat like the Stant 48799.
Otherwise, it depends on what you're looking for. I know my '05 300C was way too quiet for my taste...but the '05 has the three-pass mid muffler, the quietest and most restrictive. I'm not sure about the '07. If you have rolled edges on your exhaust tips, you should also have the 3-pass muffler. If you have the sharp edges on your tips, you have the two-pass.
If you want more sound like me, you can cut out the mid-muffler and either replace it with straight-through mufflers or straight pipe. If you just want a little more growl and a throatier sound, you can leave the mid-muffler as is and modify your resonators, it's cheap and easy.
Another worthwhile, inexpensive mod is to replace your stabilizer bar frame bushings with poly or Nolathane units, and while you're at it, replace the end links. That'll snug your handling down a bit.
WOT shifts (especially 1-2) were too soft and sloppy on mine, so I used a handheld tuner to tweak the shift firmness. I don't know that it's worth laying out $400 for a tuner for a stock first-gen 300, but if you can find a used one for $150 or so, you might want to apply an ECU tune as well.