If they do a higher power version of the engine look for differences in the intake and exhaust manifolds.
Has it got an electronic or mechanical fuel pump? (pretty sure the HDi is a diseasel engine).
make it breath better, better flowing exhaust/inlet (off the shelf cold air feed ebay special won't help) the cheap tuning "piggy backs" don't usually help either, they normally just force the car to run rich by fooling the ECU's temp readings.
As mentioned, better suspension and brakes will massively improve the feel of the car.