what SHOULD FBW be doing...
Without FBW systems, gusts are dealt with by reducing alpha to recover from stall/overstress events. Talk to a glider pilot who has encountered extreme turbulence in stubble fires! Paradoxically, whether in up or down draught, the nose ends up pointing down a little, to maintain airspeed... and keep alpha sensible.
The FBW system does what the programmer tells it to, and there's the rub....
Up in the pointy end of the envelope, control deflections must be kept below any level that will cause structural pain, and this will come into direct conflict with the movement required to give an effective control of attitude in severe turbulence. Hence the need to go slower in rough air....
When the envelope is breached, priority should be given to avoiding stress by excessive control deflections, and hopefully the ups and downs may cancel themselves out in a reasonably short time scale.
Huge cu nim have descending air round the outside, and high speed jets in the middle... any speed can result in large stresses being imposed. Gliders have been spat out in pieces, and considering they enter at around 50 knots, it doesn't take too much imagination to figure out that 450 is too fast...
FBW is maybe not quite the right description, Fly By Computer should be employed. Maybe the data available needs to be researched further..