has the keys right: "filling the fender cutout" depends on tire diameter and degree of lowering you can tolerate.
The tire diameter you select may change the overall gear ratio to reduce RPM at a given road speed; the compensation is to change differential ratios. If you don't want to change ratios, you'll be stuck with less acceleration but more leisurely engine work at highway speeds.
Lowering springs will very likely reduce ride comfort in some situations. "Progressive rate" springs are reputed to accommodate lowering with softer rates (better ride) at small compression values, but eventually arrive at stiffer rates (harsher ride) on larger deflections.
I used Steeda Sport springs for a couple years, and thought they were a good compromise. Squid recently recommended Steeda Ultralites (constant rate) as a good combination with slightly upgraded shocks and struts (GT500 takeoffs). I have no experience with either of those.
The Tokico D-specs worked well with the Steeda Sports. My current decision process resulted in FRPP "P" springs, progressive but only lowered the car by a half-inch front and three-quarters rear, pretty much the same final rate as the "K" springs (and Steeda Sports, by my senses). With the Koni Sport struts and shocks the ride is not much deteriorated on the highway, but does level the car in cornering, acceleration, and braking. No significant changes in normal ride height, so no additional speed-bump or driveway scraping.
PS: Pictures with 18x8.5 "Fanblades" in my gallery.
ADDED: 2014 GT 5.0A conv. black: electronics, comfort, security, CA plate (FLICK U); assembled 06-13-2014, delivered 09-25-2014; odometer today (03-21-2016) 4,321.5(--> DELETED --> 2013 5.06M coupe, black:: Odo LAST day (09-25-2014) 7,975) (--> DELETED --> 2008 GT/CS: Odo LAST day (10-22-2012): 9,719.3; #2075 of 2509 convertibles, #560 of 718 in Ebony. 2009 GT/CS: Odo last day (09-08-2012): 12,000.1; #34 of 1705 coupes, #3 of 377 in Performance White. <-- DELETED <--)