Wouldn't it be even better to replace the cat with a straight pipe?
The expansion/contraction that the cat chamber does will cause some unnecessary turbulence that decreases power.
Other cheap fixes are to check out so that all manifolds and joints are aligned and that no gaskets intrude into the airflow. Same for exhaust pipe - check that all joints are aligned.
A K&N airfilter is also a relatively cheap modification.
And try to figure out details that actually have an impact on drag. E.g. that the hood is adjusted right to avoid unnecessary turbulence over the fenders.
Strip out any unnecessary weight is another. - Who needs carpets?
It's all about power to weight ratio when it comes to acceleration.
And of course - there are wonderful details to be found at the Home Depot - even if they aren't intended to be for a car.
Replacing bulbs with LED:s is another fix that can go in relatively cheap. But you may want to go for an electronics store instead of the car parts. High intensity LED:s are actually relatively cheap if you buy them at the right place. And you can place them anywhere without having to worry about heat.