Hello Iestyn,
You never get something for nothing, and these boxes or chipping are no different. Whilst you may get more power or torque, you will use more fuel, and the added stresses on the engine and gearbox may cause premature failures of components.
Some boxes may produce more particulates in the exhaust that might cause MOT failures
Such devices enviably void guarantees,, and you must inform your insures that the car has been modified.