After much confusion, the OP basically wants to know if you can run the factory rear wheels/tires on the front...
The issue is the wheels, not the tires (or the tire sizes). Everyone seems to agree that the rear tires will work if installed on the front wheels. The question is whether the rear wheels will work (with those tires).
The rear wheels are wider than the fronts but they also have different offsets (so their effective/proportional centerlines might also be meaningfully different)
There are several different tire/wheel combos for the i8. My friends at iND gave me this link:
https://www.etkbmw.com/bmw/EN/search...i8/USA/36_2467
Note that not only do the wheel sizes vary but also the offsets. Take the first listed example: If a wheel width increased by a half an inch (12.5 mm), and the offset increased by 10mm, then most of that increase in width was pushed into the wheel well. Just because a wheel design is increased in width doesn't mean it happens symmetrically in both directions (inward/outward). Offsets matter.