K&N Pannel Filter
Stainless Steel Cat back exhaust
Decat
The above will give a slight power gain but nothing more than 5-8bhp.
theres no point doing anymore engine mods than that, the next thing you want to do is sort out the handling and stopping power.
if you can stretch to coilovers then go for them, if not get an Apex -60mm kit (this comes with uprated APEX Dampers and -60mm lowering springs.
A set of EBC Green Stuff brake pads are a good upgrade over standard and half the ammount of brake dust over standard pads! then if you want more a set of say brembo MAX disks arn't too expensive and are well matched to the EBC pads.
the BEST way to improve performance especially on these smaller engine cars is loose as much weight as possible (ie. loose the back seats, spare wheel and tool kit, if you want to go even further a set of light weight front bucket seats, loose the door cards etc