FWIW, reading and summarising previous posts and also historic ones,what the 2.3 petrol engine is like for economy is rather like measuring the length of a piece of string.
The 2.3 petrol engine whether in bog standard tune at 185 bhp (very early ones had 170) or anything up to the HOT's 250bhp will give you low to mid 20s round town and mid to high 30s mpg on a long journey.
Manuals will give a few miles more, (in my own experience it's not as wide as the 15% margin aljshep mentions, but we're probably both right!) but the biggest factor is how heavy your right foot is, regardless of power output.
Various after-market tuning options, apart from maybe the extreme top end stuff, also seem to have negligible effect on consumption, ignoring the greater likelihood of a heavier right foot.
I've been running a 2.3t Vector auto for almost 100k, 185 bhp originally then Hirsched to 220 bhp at around 50k.
Overall consumption before was 26.5 mpg, 25.6 after. But for the past 20k I've been doing more short journeys than the previous 80k. Worst tankfuls are around 21mpg, best was 35.9. I think my right foot is heavier than average, but my figures are reasonably typical of those others have posted.
Whatever you get, it will certainly be a lot less than the 2.2 diesel gives. But, hey, it'll be much more fun!
As mentioned, check the VED rates - they can vary enormously with age of car being a big factor.
IMHO again, a 2.3 Aero will be enough. While aftermarket tuning is great, the insurance cos don't like it - see many, many posts. I love my car, especially with the Hirsch tuning and springs (fitted 10k after the remap), but looking back I should have gone straight for an Aero in the beginning rather than a 2.3 Vector. Would have been less grief from the faceless ones.
I echo ScarbSaab's comment on the Sport button. And the convenience of an auto around town. I've never really bothered with the paddles though.
D'you need anyone to come window shopping? There'll be a dozen or more of us willing to help you choose!