Chubbs, apologies for the lateness of my reply but I don't often wander this far from the Lee Enfield forum.
The NRA when asked for proof that 155 gr ammunition was dangerous in Lee Enfield's went to ground and realised that they actually had no proof.
However LERA's secretary decided, (I'm TR Captain) that we ought to be responsible to our members and also to our collection of Lee Enfield rifles by only firing factory made 144 gr ammunition through them.
It's been admitted privately (so the rumours say) that somebody in the range office/NRA building went of at half cock and issued the statement without any reliable research and it was all based on hearsay.
My take on it is an historical one, the rifles were designed for 144 gr; 150 gr works very well in the load I have mentioned above, but just how far do you push these things...?