The i3's wheels are forged, and as a result, tend to weigh less and be stronger than a typical cast wheel.
The reason why BMW chose the tall tires is that that means the actual tire patch on the road for acceleration and braking is about the same as a smaller/wider tire in the same rolling diameter. IOW, going smaller/wider won't give you more tire patch, just orient it differently. And, in the process, create more drag and potentially impact the fender or give you problems with the clearances at max steering angles. Keep in mind that the i3 has a pretty small turning radius, and to get that, the tires turn more than many other cars (especially front-wheel drive ones since the CV joints tend to limit the available range).
A wider tire will have more drag.
Unless the overall diameter is the same, the speedometer and odometer will not be accurate. A smaller diameter assembly may LOOK like you're accelerating faster, but probably not. Plus, the stability control will likely limit that to prevent wheel spin, so you may not gain much of anything. A wider one could give you a bit more lateral grip as the tire patch would be wider (but shorter), so oriented more for turning capability verses acceleration and braking.
A wider tire, in general, won't handle rain or snow conditions as well as a narrower one.
Tread depth also impacts the rolling resistance as it gets deeper, so the as new depth may be a bit shallower, and that does affect their life. Deeper tread tends to squirm a bit more, increasing the rolling resistance and generating more heat, which also affects the design of the rubber compound. Lots of tradeoffs/optimizations. You can improve one aspect, but likely would diminish another when you play around. You might get the mix better tuned to your preferences, but may not like the consequences.
2011 535i x-drive GT, 2014 i3 BEV