My perspective has always been larger wheels look better. My plan was to purchase 19" wheels for my BRZ, and for the longest time I thought everyone saying 18's were perfect didn't know what they were talking about, and that 19's filled the wheel well better. But then I saw some 19's in person and they made the car look ghetto DUB fabulous, which isn't my style. I'm not saying there aren't any 19's that look good on the car, but I've since realized a full faced 18" wheel fits the FT86 proportions perfectly, and with a drop the car can look better than with 19's.
Aside from aesthetics, as others have stated, acceleration and handling will suffer due to heavier weight and higher rotational inertia, and the lower profile tires will provide a much harsher ride and provide less protection for the wheel. But ultimately do whatever makes you happy, not what makes the forum's happy.
19's can look overly large on the FT86...
