125D Tuning Remap or Tuning Box? best options?

Evening,

I've had my 2013 125D M Sport Manual 3Dr in EB a few months now previously had a 123D. Loving the car its real step above the older 123D in everyway especially ride quality! But with the 125D I'm looking to get it either Re-mapped or add a tuning box, has anyone already done this, what sort of power could I expect while keeping it reliable? Also what's the best route Remap or a tuning box keeping the warranty in mind?

I've see seen some of the TDI Tuning UK box quote 271BHP and 536NM are these reliable figures? would a custom remap be better?

Also how would the remap/box effect the driving modes would they still work like Eco pro?

Regards