going significantly smaller idea
 

On my 2007 Civic Si Coupe, I had dealer-installed Axis Widetrack wheels (different lip size front vs. rear) and 235/40-18 tires. Loved the look but after a few years of that I was ready for something lighter.

Stock was 215/45-17 (just like the FR-S). I went right past that down to 205/50-16 with black Motegi MR125 wheels (15.2 pounds each). I found the handling to be a little less planted, requiring more steering wheel input to make the same turn (slip angle difference?) and having a bit more of an unsettled feel. Just something to get used to.

However, acceleration was a bit better (due to saving quite a bit of weight (34 pounds / corner vs. 48 pounds / corner) and also having a smaller overall diameter for the drive wheels), fuel economy increased by about 1.5 MPGs, interior noise was much nicer, and reaction to bumps (compliance) was much improved. I liked the trade off and so did my main passenger. Car was a bit lower and visually smaller as well.

Considering the same sort of thing for the FR-S. Haven't looked into bolt patterns, offsets, weights, tires, selection, etc. yet. Just my back of the napkin musings, what do you think:

Stock: 215/45-17
sidewall = 3.81
total diameter = 24.62

Front: 15x6.5 + 195/55-15
sidewall = 4.22
total diameter = 23.44

Rear: 16x7.0 + 205/50-16
sidewall = 4.04
total diameter = 24.07

In Australia, the GT ships with 16" wheels (the GTS has the 17"s)
The wheel and tire spec are
205/55R16, 16x6.5

I don't think 15s in the fronts will clear. The stripper RC in Japan has 16s steel wheels all around.

There are a number of 15" rally wheels that will fit over the Subaru 4-pot calipers (which use the stock rotors).

My car came with 16 inch alloys (standard spec on the manual here) and I must say that they ride very well and grip OK.

Sticking on better rubber would probably fix up any grip issues.