The thick chassis gives three main adavantages as far as I am concerned.
(1) More consistent grip, i.e. grip throughout a corner is more consistent as there is less flex.
(2) More reactive meaning the car responds quicker to direction changes. This is especially usefull on tighter tracks.
(3) Responds even better to setup changes. Due to the lack of flex changes in springs, oil, geometry etc are even more noticeable.
The last advantage also means the thicker chassis requires a bit more work to get it setup correctly for each track. I have found that usually only minor changes are needed, but if you don't make those changes the car can become a handfull to keep under control. It will still be fast though.
You are right that with the same setup the thick chassis will give less grip than the kit chassis, but by using one step softer springs and possibly a 1mm shorter wheelbase you will have a good starting point that will have sufficient grip. Even on low grip surfaces the chassis can be an advantage.
I definitely think it is an advantage for the mere mortals amongst us (I am one of those
). However you will in all probability need to take some time to find the right setup for you and the track(s) you race on. If you do you will find the car is faster and actually easier to drive as it will be even more predictable.
. Corrected the post.