Hi Andy,
Welcome to sidc mate. Nice to hear from someone else round here who's got a cat[H] I've got number 009! had it for about 18 months now.
So far mods wise i've got a green cotton panel filter and a scoobysport back box, but there should be more coming soon[6]
The green cotton filters are supposed to be good and don't cost much. Maybe that and centre decat pipe would make a noticeable improvement without forking out loads.