Hi bud, the first and pretty simple answer is that any such power gain from intakes and exhausts are cancelled out by the ECU. Its cosmetic/sound only.
Cold air intakes are available for about £85 upwards. Exhausts from about £400 upwards. All a matter of choice of sounds.
Grounding kit improvements, different members have had differing results but none bad.
If you are looking to upgrade the ECU do a search on here, it has been discussed too many times to link.