I know what you mean by the fuel consumption , my mate has just got a 53 plate M3 not sure if that's the same one as you? With smg box and he is getting around 17 mpg !
Will be the same model - mines a 55 plate with the SMG box and 17mpg is an easy reality if you drive like you stole it all the time - I would say that this could even be a lot lower if you try really hard!
The problem is the noise it makes past 5k - addictive is not the word