Michael, you'll probably gets lots of opinions on this, but I wouldn't spend much on trying to upgrade the performance of the basic 1600 motor - certainly not the cost of a high-quality exhaust.
If you're unhappy with the power (and torque) you've got now, maybe save your pennies for a larger, custom-built motor. The stuff that does make a difference on any motor - better crank, cam, heads, carbs, and exhaust, etc. - is expensive and, within limits, costs about the same on a two-liter engine as on a 1600. So, once you go looking for more grunt, it doesn't make a lot of sense to limit yourself to 1600cc's.
I'm not one of the resident engine gurus, but I'm sure they'll be along anon.