Engine Performance problem
2004 Renault Laguna 4 cyl Front Wheel Drive Manual 74000 miles

I have recently had work done to the engine there was a lot of white smoke being produced and this was sent to a garage and they have replaced the piston rings this has fixed this problem, however my car is now low on power and it appears the turbo kicks in at around 3.500 rpm and it a diesel and need to change gear at 4250. what is the possible causes of the low power output and how can i fix them