It is important to remember that the Amel tackles are vangs to pull the boom down and control sail shape, and are really very poorly set up to function as preventers--no matter what Amel calls them.
Anytime the boom is further outboard of the end of the traveler, we tension the vang to control sail shape and keep leech tension up and reduce excessive twist. The boat goes faster with the whole sail, top to bottom, trimmed to a proper angle to the wind
In strong winds and rough seas counting on a line connected to the middle of the boom to stop an accidental gybe is a very bad idea. Never on an Amel, but on other boats, I have seen lines rigged that way cause the boom to fold in half in an accidental gybe, or when a deep roll caused the end of the boom to dip into a wave. That ruins your whole day.
A REAL preventer is attached at the outboard END of the boom and lead forward, then aft to the cockpit so it can be eased--under control--if needed.
We never sail wing-on-wing on Harmonie. If we are going downwind, the ballooner is set. If we are not going downwind long enough to justify the effort of the ballooner, we reach at 150 degrees to the true wind and then we make better VMG downwind than we would wing-on-wing. If we really need to sail straight downwind a short distance for some reason, I furl the main and go jib-n-jigger. We might lose a little speed, but for a short distance, the peace of mind is more than worth it to me.
I personally would never sail wing-on-wing with the main boom anything other than fully out to the shrouds. An accidental gybe happens when the boom crosses the wind. If the boom is not all the way out--as far as it can go--you make an accidental gybe that much more likely. It is not worth it.
The number of boats I have seen seriously damaged by an accidental gybe has taught me to avoid them at all cost.
Norfolk, VA, USA