You are more likely to be travelling at less than 35mph than 70mph. Most guys will be doing residential properties and so most of their time driving will be either trying to get up to 35 or breaking to take the next turning.
Admittedly a 70mph crash test would give you more confidence in the system but a few other things would need to be taken in to consideration in a real crash.
1. Inertia. Your more likely to hit a car than another van and as you have the added weight of a tank of water in the back then your vehicle will have more forward force than a car or half loaded van of the same size and so more forward force means you would take longer to slow down than the vehicle comeing the other way. If you hit the back of another vehicle then you are more likely to push that vehicle forward and so again longer to come to a stop.
2. Crumple zones.... The test doesn't take in to account these safety features either, which once again help to reduce how quickly you actually stop and disipate energy away from what is inside a vehicle including you.... Many deaths in vehicles are not from external injuries but from internal injuries caused by the vehicle stopping to quickly and your insides carrying on forward and hemmoraging.... (sorry if your eatting).... see top gear crash test of Smart cars..
3. Not many people will carrry 1000kg tanks and I'm sure the Brodex 1000L tank would have extra fittings and so the fittings to bolt ratio would actually be less than the 650 tank.
So to me it looks like Brodex have gone and crash tested for the most likely crash.
I will admit a 70mph 1000L full system, with poles, reels, etc, crash test would be good for even more confidence but if your mainly local and use a full 650 or less tank then that test should make you feel safer.....
Regards
Mr H