the most important dimensions in exhaust design are 1, length and 2, bore.
Length is critical as it is tuned to the distance one pulse of exhaust gas will travel before running out of energy and creating "pull" or a slight scavenging effect which speeds up the gases leaving the next firing cylinder.
Bore is a variable, and determines the back-pressure in the pipe (thats how fast the gases can move down the pipe before it starts to restrict the gas exit speed - if you take a straw, and blow down it gently, its easy, but blow harder, theres a limit where you can blow as hard as you want, it just wont come out the end any quicker and feels like its hard to blow, that pressure is back pressure) from small and slightly restrictive which gives good mpg and torque low down in town (like a standard exhaust) and up to oversize, which gives top end breathing but reduces low-rev torque and mpg.
To better the standard exhaust is easy but its a compromise. Silencers on non-cat cars were the first point the gases could expand, and an expanding gas will cool rapidly as it expands and slow down, quietening the sound it makes. Hotter gases move faster.
Now, a cat equipped car, has the cat as the first expansion point, as well as the honeycomb rare-metal insides to blow through, so a bit of expansion happens here, but the catalyst heats up to burn excess fuel particles in the gases away, so slightly speeds it up again, after that anything you do that leaves the cat intact will only have a small effect on bhp etc(5 to 15 bhp max, and maybe even nothing!), and a big effect on noise.
We fitted a new exhaust in 1993-ish to a 91 astra 2L GTE that ran 130 bhp as standard, and after fitting a shiny £150 sebring (at the time it was dear!) - it dynoed at just 131.4 bhp - a paltry 1.4 bhp for £150, and that was a big chunk of a weeks wages back then!
It felt a bit faster but probably just cos it sounded sporty, and it was quick anyway, but it wasnt much different in reality.
Its down to what noise you want i reckon