Most 16" rims will fit (although some high-offset ones won't!), but I'm hoping that either the super light Kosei K4R (under 11 lbs.!) will clear the giant Leaf brake calipers, or the still lightweight 13.5 lbs. Kosei K1 will. The reason a 15" is preferable to a 16" is that 15" tires (for the same outside diameter) are lighter than 16" (the lightest I've seen is 18 lbs. for LRR tires vs. >20 lbs. for 16"), and so there's further weight savings. Plus the weight is distributed more evenly throughout the radius for less flywheel effect (worst case scenario are huge rims with thin tires).
So on paper, if 15" Kosei K4R fit, it would be just shy of 29 lbs. total for the rim/tire combination vs. around 42 for the factory rim/tire (roughly 21 rim and 21 tire). That should not be an insignificant difference.
The big advantage of 16" is they look slightly nicer and I wouldn't need new tires... yet.
Cheers,
Tal