To be honest there is no point in you changing the whole exhaust system if your going to stick with the 1.1 engine. I know the only reason Dan has fitted the Ashley is because he is upgrading the engine shortly and the exhaust will suit.
Long story short just change the back box if you want a better exhaust note. You wont get decent performance out of the 1.1, no noticeable power improvements so save your cash instead of going full system.