Hey man, i have never run a KB on a mustang, but i had one on my lightning. The KB is a great blower, their customer service can at times be frustrating, but they do perform. I do not know what options are available for the 5 liter, but if you can, get the biggest displacement KB you can afford. The only reason i say this is because, at some point in time you are going to want more boost, its very addictive, the smaller blowers work well if the only thing you are going to do is run it on the street and the occasional track pass. As for my KB, it didnt really "wake up" until i hit about 18psi of boost, and at 24lbs, it was an animal. I dont know how the 5 liters react, but if you are cruising at highway speeds and lay into it, you dont see "full boost," it acts just like a centri blower. If you have a centri blower and lay into it hard in 1st gear, most centri blowers i have had will start making boost about about 2000 rpm, and then you just watch the tach climb until redline. Both blowers are going to make great power for a street car, but i think that a lot of owners sway toward the centri blowers because there is the ease of install, most tuners are familiar with them, and you can achieve awesome power with them, and also have had better customer service with most of the centri blowers i have had. Just my opinion, hope it helps you some!!
Anthony