I finally got around to having the second hand M50 dual mass flywheel and clutch kit fitted today.
I can confirm that it is a bolt in replacement with no modifications required.
The comparative weights are 16.5kg for the M44 parts and 18.0kg for the M50 parts, which I hadn't expected as the weights shown online for the M50 parts worked out less than the M44 parts.
So far it is driving great with no noticeable effects from the heavier parts. The old M44 parts showed signs of abuse... The friction material on the disc was cracked and there were burn marks on flywheel and pressure plate.