I drove my friend's FiST with 15s (I think 205/50 or .../55) with summer dunlops ("z" something, I think), great ride, but suffered quite a bit on turn-in. The steering slowed considerably, and braking suffered... at least compared to my FiST with the stock rubber. YMMV
2017 Magnetic FiST w/Recaros
Just swapped my 15x7 Sparco Terras with 205/50R15's back to my 17x7 FRPP "Rally" wheels with 205/40R17's due to a TPMS error in one of the 15" wheels.
I can say without a doubt that the 15's dumb the steering down quite a bit but braking is better with the lighter 15" setup.
Both wheels/tires have 200tw tires on them so its not a rubber issue. The lighter 15" wheels have less rotational mass on the hub/rotor for the caliper to clamp down on versus the 17's.
As far as turn in, its drastically better with the 17" wheels, even if they are heavier. It feels much more precise on initial and on follow through. So as you turn the wheel, the tire doesnt need to catch up to your inputs throughout a turn. The 17's make the car a point and shoot while the 15's need some input while you transition the wheel to keep the front end placed where you need it.
As far as tires, there are almost triple the performance tire selection in the 205/50R15 versus the 205/40R17 stock size. The shorter sidewall also changes the gearing quite a bit and you can really feel it when accelerating from a stop and out of corners.
To me the 15's are the way to go over a 17" setup as far as price, selection and weight. For steering feel and overall precise-ness, its the 17's hands down. Ill still continue to run the 15's though as the weight drop is significant over my 17" setup.