I'm not sure if flex gives more grip but it certainly gives more compliance. In the low grip I race in it's difficult to put down mod power with a super stiff chassis. It can be possible to tune the suspension so the car is driveable on power but that usually compromises the car elsewhere on the track, so a chassis with a reasonable amount flex is best. I've found the same thing with the stiffness of the suspension arms, particularly in the colder months.