The biggest waves I have sailed into would have been 2-3m caused by 20 knots of breeze against 2 knots of current. Hard on the wind, both hulls met the seas at pretty much the same time. A much more pleasant motion than the corkscrewing on a cat.
there is a discussion on proa file international about the unintentional rounding up of supposedly Harryproas while sailing. I am not a member of that list but the explanation offered there is stalling of the rear rudder at slow speeds. That certainly makes sense particulary if you try to sail too close hauled. But I see another reason which I deduct from my very limited experince sailing my small mono over steep 2-3 m waves. Once you have just passed the wave crest at some speed it is very easy to turn the boat as what is left in the water at that time is part of the keel and my very deep rudder - whereas the front third of the hull is in the air. Usually I turn the boat down hard to avoid slamming. What would happen with a Harryproa sailng at 45 degrees to the waves? The first third of the super sleek lw hull would also be up in the air and the front rudder either raised or up in the air too. But at the same time the bow of the rather fat ww hull with its superstructure could get hit hard by the very same crest. This would transer a strong impulse into that hull inducing a possibly difficult to control luffing tendency.
Unfortunately I have nor read any reports of a proa - no matter what kind- sailing in such conditions and certainly not seen videos. All videos I am aware of seem to be showing "flat" water sailing under the protection of a close by shore.
Heinrich