In ref to the exhaust its worth mentioning that smaller engines need a smaller bore pipework exhaust to produce power as they need more help to make back pressure - which is what helps produce torque.
If you was to have a too larger a bore exhaust with say a straight throught Jap style can this would probally lower power than add it as the gases are exiting the system too fast and can not generate back pressure.
My best advice is probally to keep your cat, lose the centre box and then just fit a nice sports back box like a Peco or Sportex as they are cheap, Id avoid large bore straight through cans for the reasons mentioned before.
Somepeople will say its not worth wasting money trying to tune a smaller cc engine but I dont agree, as long as you dont go silly with expensive stuff - its what you learn on your first few cars (normally small cc) that you take onto bigger more powerfull cars