Warranty issues aside (I would never condone risking a BMW warranty) I would hate to own a diesel that wasn't remapped - Its not about headline figures, but driveability and feel. The difference is night and day.
I find BMW diesels to be very 'dull' pre-remap, even the most powerful ones. I had my 535d remapped - had more than enough power as standard but didn't utilise it in a way that made you feel like you were going fast. After the remap, it pulled harder and from earlier and the throttle was more responsive. It also ironed out the flat spots.
My 320d is now remapped - and although this is a car that does actually need more power, what I really like now is the way it drives and delivers the power, not the top end hikes.