Really wouldn't bother with a decat on the downpipe, its a faff to change round come MOT time as it will fail, just get a 200 cell sports cat and all's good. Hardly any restriction and will pass an MOT.
As for modding your car, whether it be Show or Go, everything can be changed or Retro fitted, have a good look at some cars in the Members Rides and you'll get some ideas.
Change all the interior window switches and Headlight Switch. All simple to change apart from the Fuel Flap switch, that ones a PITA.
Also look into changing all your interior lights to LED's. Costs around £20 ish for the full set and a simple change over. Have a look on here, there's a thread by member Monte re LED lights and have a read and buy from there.
You could change your climatronic unit to a Mk6 one, add a RCD510 or RNS510 Headunit and change your cluster to a Highline (if needed) or a R32 white dial cluster. Or even a Mk6 MFSW steering wheel.
Little bits, like a shiny dead pedal, seat inlays, brushed aluminium ashtray cover etc etc etc.
Door warning lights are a VERY good mod, really useful and get noticed by peeps who know they're not normally there.
@OP, My review/write up on Door Warning Lights......
~~ Clicky here ~~If that's not enough, vinyl wrap, paint or dip all the interior trim/parts. It never stops.
Happy modding!