I used Castrol SRF & Hel braided lines with Carbotech xp10 on a set of Brembo standard replacement discs on my, now gone, track day car. It wasn't a FSTI but that combo was a huge jump from regular lines, fluid, standard disks with Performance Friction pads (which are supposed to be good). Wish I had enough experience with the FSTi to offer some guidance.
However, the reason I did that was specifically for track which you said you won't be doing, but yes there was more feel and response (good fluid lines and pads will do that) as well as a lot more bite/power but importantly it meant I could stay out for 45 minutes to an hour (until the fuel light came on) without suffering any brake fade or drop in performance. The one thing you want to have on track above everything else (in my view) is consistent braking, otherwise how else do you improve?
On the road, hmm, well unless you are going at 10/10ths for an hour straight without coming across any traffic (if you are please PM me the road!
) the only thing that should matter is feel, bite and stopping power. If its just the current setup doesn't give you enough feel but stops good enough I'd go better fluid and braided lines (as old rubber can stretch), if you want more power then by all means add stronger biting pads and discs that can deal with the heat. Only thing to bear in mind if you do that is you may get more noise with an aggressive pad, not be as good when cold and potentially have to deal with a lot more wheel eating dust than before so will need to keep on top of cleaning.