I had the same thoughts, test drove the 2.3i and felt it needed more power, the 3.0i is more money however the calculated depreciation was less and was thus not that much more to lease. By the time I had added on the extras that I got with the 3.0i, it was actually more expensive to lease the 2.3i than the 3.0i. Optional extras have a much lower calculated resale value (for leasing purposes) than the car, about 10% of purchase price after 3 years.
If you look at BMW used cars the 3.0i remains £4 to £5k higher than the 2.3i
I love my 3.0i Zed, it is fast enough for me, gives good fuel economy, same as the 2.3i and you don't see many of them, not sure how many 3.0i E89's are on here, but certainly not that many. Is Petach the only other?