We have part of our ball manipulator stuff on the kit chassis, but we're planning to make a custom chassis. The kit chassis won't work for what we want to do, and we've built a custom chassis every year except last year (and the kit chassis does have a design flaw that we didn't anticipate, and led to our robot failing in eliminations).
I guess what you need to do is figure out where you are, and where you need to be, and decide if you can get there using the kit chassis, or if you need to do something else. Also consider just modifying the kit chassis, making kind of a "hybrid" chassis that uses as many existing parts as possible, but new parts where needed to make it work.
Another thing....we try to make the "upper" part of the robot as light as possible, and we don't worry much about how heavy the chassis is, within reason. Our first year, we had a top heavy robot, and learned the hard way about center of mass.
This year we plan to make another steel chassis, it's pretty entertaining watching people see it and shake their heads in disbelief
