Are you happy with the way your car sounds?
If not then i would go cat back.
If you are, seeing as you have replaced the mid pipe and have a drop in filter along with FRSU and assuming you have some decent tyres on.
I would go down the route of upgrading the brakes.
Decent pads, braided lines and some good fluid will come well under £500 and have a massive impact on how well the car stops.
If brakes are all good then I would personally get a k100 board put on with the re flash (Hondata map 4)
This will come under £500 without rolling road (yes many will say dont) but give you a good punch of midrange and a little more top end