I would say it has little to none flex. It's extremely stiff. I also opt. the strengthen chassis supports. so it makes the car very stiff. And those aluminum bulkheads that is holding the car together are pretty thick also.
RCMonk.
I think you can still get the saddle pack chassis from Harmony Hobbies in Hong Kong. you just have to write them a email and see what happens. Go check the Hong Kong thread. They have the contact info in one of those threads.