I tried the Kit chassis, a Berserk 2.0mm chassis, and the Samix 2.3mm chassis. I also have had made a Bersek 1.60mm, 2.00mm, and 2.25mm Top Decks.
I've only raced mod. Hobbywing 3.1V esc and Hobbywing V10 4.5t motor.
I couldn't comment if the Samix chassis will be better in the Spec classes. We need someone to try and report back.
In respects to flex its hard to define, but if the Kit Tamiya chassis was rated at a flex of 10 (hard), and the Bersek 2.0mm was a flex of 5, then I'm guessing then Samix 2.3mm chassis is a flex of 7.5
As said before, the faster you go the more chassis flex is required. the slower the car the more rigid the chassis needs to be. Of course there are exceptions, conditions, and circumstances that are contrary to this, but I've been very happy with the Samix thus far. Another factor to be considered is the Samic Motor Mount. This mount used 4 mount screws and are considerably narrower/more central on the chassis.
The Kit chassis I wrenched and wrenched on that thing. I could get it to steer and to have rear grip but could not give it cornerspeed. The Bersek 2.0mm was way better but stalled mid corner (too much flex). Samix seems to be the winner out of the ones I have tried (not saying there isn't another out there that could work just as well or better).
Heck, it seems even Kiyo has found a decent chassis for the TRF boys now (thanks goodness!!!, lol)