Your only options for the 6 cylinders are the 23i (2.5ltr non turbo) 30i (3.0 ltr non turbo) - 35i - 35is (3.0 ltr twin turbo) The 18i 20i & 28i are all 2.0ltrs turbo in line 4 cylinder of various BHP and being turbos they are very tuneable to increase performance. As the 23i & 30i are non turbo there are few gains from tuning.
I have the 23i and its adequate, if you rev it, it likes to live above 4000 rpm for any serious performance but its not pin you into the seat performance. If you want instant power I would go for the 3.0ltr versions, the 35is being the performance model.
Not had any roof issues to date (2 years) but they can be problematic if you read the forums, usually regarding the wiring loom between the roof and the rear clamshell.
