I would think the impact would be massive as the forks only have a few inches of movement before they bottom out....
I entered a trial once on my M20 and that more than found the limits of the forks performance without actually leaving the ground...At best I only consider myself as an enthusiastic amateur when it comes to off road riding and found the whole exercise exhausting...It only served to increase my admiration for the 'average' DR...Ian
I used to do vintage and classic trials on my 1927 Raleigh and also on a 1927 model 25 Norton, later I rode in grass track and sand racing on a'39 Speed Twin and a garden gate Manx with girders. I never found the girders a problem in these events, in fact they gave more positive steering than some of the weaker early teles. None of these events involved any jumping at any sort of speed though, maybe the odd wheelie which gave the forks a good jolt on landing.
Picture those vintage and pre-war photos of rigid girder fork bikes leaping over Ballaugh Bridge and landing front wheel first at maybe 60-70 mph, the riders were iron men, real heroes, just like our DRs in all conflicts.
Maybe these sort of photos explain why we see WD Nortons with stretched fork springs and dented tank fronts, I can't imagine jumping ditches on a rigid girder forked bike, must have been a very heavy landing.
Without the rebound rubbers on the fork legs the forks bottom out when the mudguard hits the underside of the steering column. My 16H used for pre-65 trials bears witness to this event.