Most lowering springs are going to be stiffer, BUT, they are not all the same. You will have to check the spring rate on the different springs you consider. I have the Hotochkis springs, only slightly stiffer than stock, and rated to be able to be used with stock shocks and struts. Others are stiffer. The other thing to keep in mind, you can go with stiffer springs but still not have a harsher ride. Depends on what you us for dampening, aka struts/shocks or if you went to coil overs. (although, sounds like coil overs are not in the consideration). There are people, and I am sure some will eventually chime in, running noticeably stiffer springs but combined with upgraded struts and shocks that can handle them, so not a stiffer ride.
2013 GT 6 spd, GT500 mufflers, BOSS quad exhaust, Roush CAI, Barton shifter & 2 post bracket
CS GT-40 Wheels, Hotchkis sway bars, upper and lower control arms, springs (1.1/1.6 drop) and adj panhard bar KONI yellow struts and shocks J&M caster/camber plates,
BMR Upper control arm mount and lower control arm brackets.