SOTP is all so subjective, it's hard to say if 15-20 HP will make a difference on a 300 HP car. The biggest thing you will notice is the sound - assuming that you opt for an agressive sounding cat back system.
When I changed out the stock exhaust to Corsa on my modified 92 LT1 car, I dyno'ed it before and after to make sure. The answer was a 22 HP gain and a much louder car. To me if certainly felt better. Then the sound alone probably had a lot to do with that.