Hello from another newcomer, so don't feel I can truly welcome you yet!
I also had a TT until Feb last year, and I'm now trading down from a Lexus RX to the C Max. But it's a great drive in the Ford and obviously much more economical and less CO2.
We have a dog too - I'm hoping he appreciates the lower boot lip!
Can only comment on the diesel upgrades from my experiences on the Honda Civic forum civinfo.com where these were popular - and they are widely advertised in mags like Autocar. I'd check what impact it will have on your insurance too! Must admit, had thought about investigating something for my petrol 1.8 too, but not in any rush.
Hope you enjoy the new wheels.
2008 model 1.8 petrol C-Max Zetec in Avalon Blue - arrived 14 May 2011. Also own and drive: 2005 Mercedes Benz SLK350 and wife\'s Fiat Panda 1.2 skydome.