well, basically, if you are looking for a decent sound system and worried about loosing leg room, then kicks are not for you. But...if you are truly appreciately of good sound, and want to really enjoy the music, the kick panels are one of the few ways this will come true.
the goal of any stereo system is the realistic reproduction of the recording. Now, lets think about it this way, with stock locations, you are very close to the speaker on your side of the door, and quite far away from the other side. So what results is that the path different between your left speaker and your right speakres causes the music to shift to the side you are sitting on. For example, the voice sounds like its centered around your side, even though both speakers are playing.
now, with kicks, you gain a few benefits:
1. the path difference is minimized, meaning you will obtain good imaging. you cant imagine how much fun it is to listen to a classical concert recording on a well tuened system with kicks, where you can literally place where each instrument section is on your dash...(the violins are above the steering wheel, the horns are towareds the airvent on the apssenger side...etc etc), and listening to normal songs, you will notice things you didnt before, the voice will sound like its coming from center of the dash, with insturments around the voice...the music sounds alive...
2. with ths speakres facing YOU instead of your feet, you usually get smoother response and better highs(though its not always the case with all the sepakres), things will sound more open and warmer this way.
3. witht he kicks, you get small sealed enclosures instead of open door panels, so you get better midbass reponse out of them. world of differen in my opinion.
as for hard cornering, i have seen competions cars that also double as scca racers on weekends, you basically get used to the new foot positions, kicks are quite sturdy and its no problem resting your foot against it...
so in short, your questions is sorta like someone who likes luxury and comfort asking, "why in the hell would you put low profile tires and big rims on a legend, what would you do when you go over bumps and cobble stone roads??" people with different needs go for different hings, as for kicks, its one of the only ways to get audiophile quality sound into a car

whcih is why if you go to a sound quality competition, you will see well over 90 percent of cars with kicks
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