Big discussion about this years ago on the screwfix forum, which led to me and one of my guys to doing a test on mounting a type 22, 2400mm rad in my garage on the cheapest plasterboard from
wickes, timber frame style, studded it at 400mm
Used four brackets, as per MI and 2 toggles per bracket, not huge ones either, 50-75mm probably, nothing into the studs.
Dry weight of this rad must of been 80kg+, hung it and was all fine. Filled it up, which would be another 20-30kg of water and it was still fine, i stood on top of it in the centre, another 85kg and it still didn't move, we probably both could have got on it, we didn't because if it ripped away we would of cracked our heads.
We ripped it all off in about 10 seconds mind, but the point being the board can take vast vertical load (that is not the correct terminology, downwards force etc)
[/i]