You're sport on with an exaust system get a sports cat as well for the full effect. Leave a remap to last though for the best results. Panel air filters would work well -keeping economy and giving a little more top end grunt.
Think about doing the cams to a profile of about 245o and adding NoS - both Modifications give a lot more power for relatively little money. A proper remap will lift the rev limiter as well so you can hit the redline! Weight reduction should also be considered as these cars a heavy motorway cruisers. If you can get a lower ration gearbox you will improve your accelleration as well.
I hope that this little lot helps get you started off
Engine swaps are also worth looking into and if this floats your boat then save your money for a nice M series engine and put that in instead!