A tuning box or remap! remap more expensive but better! i believe the 115bhp goes to about 135bhp but a big increase in torque! you can also ajust turbo pressure by putting an inline bleed valve from the top rubber pipe ontop of the turbo actuator! but i wouldnt recomend this as if it bleeds too much air the turbo can produce too much boost and blow it!
(THE ENGINE THAT IS)
personally a remap best route by far! anything from £150-£400