A long exhaust system wil give more torque but less top RPM
A short exhaust system will give more top RPM but less bottom torque (=poor acceleration)
And then there is a middle manifold to get a bit of both.

I use only the long manifold because most tracks are made to accelerate from corner to corner, that tiny bit less speed on the straight is pinuts compared with the gain on all corners. With changing gears you can correct the top speed.