One thing that I have noticed with using a stiff chassis (T3 - EU) is that the suspension has to work much harder, meaning overall it has more movement. This does not make a car slower, but harder on tires. Knowing how to adjust the cars roll stiffness by way of roll centre and weight transfer is key and important to ensure that grip is at the correct level, so that you can maximize corner speed and match track conditions.
The challenges with a stiff chassis, is that I found that a suspension can take quite a beating, and may require filling shocks more often because they leak shock oil. If a shock(s) start leaking (as most commonly do), they get unbalanced left to right, and the result is that you will likely be slower. This is where a racer needs to check and rebuild shocks regularly.
If you have a flex chassis, I believe it can hide / compensate for leaky shocks, and possibly compensate for a wheel that is unbalanced.
Put an unbalanced wheel on a stiff chassis i.e. front left, and it is likely that the front right end of the chassis will be affected by the vibration. The unbalanced wheel will also add unwanted movement to the suspension (high freqency oscillations), and could cause a shock to leak prematurely. The faster a car travels down a straight with an unbalanced tire, the higher amounts of vibration will occur resulting in loss of contact patch. Top level drivers pay attention to this by "listening" to the feel of the car while it is on the track, or they can visualize what is happening by placing themselves inside the car.
Basically, I see flex as a way to put a band-aid on a bad suspension setup, as it takes time to learn what works to match track conditions with different ambient temperatures.
If you don't have the time or patience to play with shock settings, or don't take the time to experiment with shocks, you will never know how good a chassis can really be for your driving style.