A sports cat manages to do everything a standard cat can do but in a much more sleek, efficient package. The honeycomb construction within a standard cat is designed to maximise the surface area of the catalysts while keeping their overall volumes low due to the sheer expense of the precious metals used.
Unfortunately, the honeycomb can be seen as an obstruction for the exhaust gasses which need to exit to the surroundings as quickly and as efficiently as possible. A catalytic converter therefore slows the exhaust gasses down, making the engine work harder as it struggles to breathe out the exhaust gasses at the end of each engine cycle. The honeycomb structure within a catalytic converter is also capable of falling apart from overheating, thus creating a further blockage for the exhaust gases to build up against.
The main aim for an exhaust system is to get the exhaust gasses away from the engine in a smooth and swift fashion, with restrictions and expansions like a large catalytic converter only causing turbulence and therefore disturbed, slower airflow. Sport cats do their best to solve this issue by decreasing the chamber size and creating a much smoother surface within the chamber to allow for the gasses to flow quicker and easier through it.
To keep emissions down, the ceramic matrix within the honeycomb construction is much finer than in a normal cat, thus making sure that the catalysts can effectively convert the relevant nasties to where they need to be to meet emission rules. Small increases in power can be achieved by switching to a sports cat, with the engine cycle being freed-up to maximise its efficiency.
Is a decat a viable option?
The straight answer in most cases is no. Many petrolheads out there decide that the catalytic converter is producing a severe blockage within the exhaust system that needs to be eradicated and therefore delete the cat from the system altogether, replacing it with a straight pipe. Although this will ramp up the volume of your powertrain and potentially eke out a few more horses from your engine, in most countries, driving a car without a catalytic converter is deemed illegal.
Cat stripping is also another form of decatting, which is the process of removing the honeycomb innards of the cat to simply reduce the amount of restriction inflicted on the exhaust gases.