In reply to the MOT question - I've got a complete de-cat, a 3" exhaust with just one rear box, and a very very smokey map - and it passed first time!
From what I understand, as a general rule with diesel tuning, the less restriction in the cat / exhaust, the freerer & quicker the turbo will spoil up. I guess on a turbo which happily reaches it's 180BHP limit without it, unless you're planning on upgrading to a hybrid turbo, there may well be little point.
T