When you say power boost, do you mean a tuning, or a tuningbox?
I don't have a diesel but have heard good things about the millers stuff from others!
Cant say I have heard much, but I have to be honest, you probably could do a better job with an air filter replacement? tenner or less from euro car part!
A good service is always necessary anyway, but a good service makes the car easier to drive and thus restores power. Diesels can boost power with a tuning box (they are cheap at about £65 for older DTUK units) up to super expensive several hundred quid units.
Bluefin remaps are good as well, especially as you can swap between stock and remap any time :D