There are several advantages an Invidia down pipe offers for a sport compact. An increase in car exhaust performance that allows for more horsepower is the primary goal, but a throatier sound and a higher-profile for your car are also achieved. Drivers who enjoy racing their cars in legitimate competition know that in order to perform at an optimal level, the exhaust system has to be able to shuffle gases out as efficiently as possible. They cannot be held hostage to restriction. With a cat delete pipe, these drivers get the edge they are looking for to go the distance.
A cat delete pipe is just what it sounds like it is - it removes your car's catalytic converter. A lot of serious custom car owners get rid of their stock cat because it is responsible for adding unnecessary restriction within the exhaust system. What a cat delete pipe does is provide a more direct path for exhaust to flow out of with less dramatic bends in the pipe. With a catless downpipe in place, exhaust flows with greater freedom and movement, which gives your car's engine the added air it needs to push out more horsepower and torque. It is an important piece in any performance exhaust system.
The shape of a catless downpipe helps tell the story more completely. A catalytic converter has a catalyst bookended by two pipes. Exhaust has to flow through the pipe before it reaches the catalyst, where it is filtered to emit less harmful exhaust fumes. When combined with the angled bends of the cat pipes, the catalyst, and the process in which exhaust flows through the catalyst itself, exhaust has two middle points it has to reach before exiting. This causes restrictive airflow. A catless downpipe bypasses the cat with wider 3" diameter tubing with fewer bends that are more subtle to let exhaust gases be removed without having to hit a detour.
There is not one umbrella answer to how much horsepower a catless downpipe brings. It can depend on how old the car is that the Invidia down pipe is being installed in. There can be as high as a 3.5% increase in horsepower, especially if the converter that was removed had been in place for many years. Most often, drivers say they received an additional 3-4 HP at the wheels after installing an Invidia down pipe, but that number climbs much higher when combined with other turbo exhaust parts.