I don't think you could see any noticeable difference between 275 and 265, or 245 and 235. Some 245/40 tires will not fit up front, so one needs to be careful there.
Wider tires do not necessarily have more contact, and certainly do not always enhance cornering or handling on stock vehicles - TireRack has some great tests. In general lower profile, wider tires have a small, positive effect on cornering. In the case of the i8, I think there is a significant effect on performance, because tire choice by BMW was slanted towards fuel economy.
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