Why bother bout fuel comsumption if u interested of 3.0l ? tell me, which big cc car can fuel saving ??? answer is none, cause its natural. I was driving 328i, now 2.0. As many drivers said, city driving = makan minyak for 3.0 but not on 2.0. Eventually, when u driving highway, bigger 3.0 is jimat than 2.0. The most important is no matter how u calculate also u wont get the actual fuel ratio. It is because, every drivers have their own modification on their wheels, air filters and etc that could effect the fuel comsumption. The most important issue could be driver's attitude. aggresive or old man type.
As i believe, an 2.0 chasis could be different from 3.0. Weight, bushes, bars, suspension and etc will be different cause 3.0 is much more heavier due to bigger block. I strongly advise u to sell off and buy 330. You are not driving japanese car that can simply upgrade engine, this is BMW. To upgrade it, there are many consideration. End up the money u spend for upgrade is actual the money where u can buy an 330.
Anyway, this my oppinion, those who doesnt agree, please not angry
Cheers !
I have almost 20 years of experience playing with Jap or Euro cars