Here are some details about X51 ( has maybe been posted before..)

Optimised cylinder-head
Admission in aluminium ( tubulures d' admission en fonte d' aluminium)
Air filter box in carbon for better look
Modified exaust sytem
Sports exaust
Extra water cooler for manual gearbox
New electonics
X51 also available with Tiptronic!!!!!
381 HP
415 Nm
Max speed: 300km/h ( tiptronic 296km/h)
0-100km/h : 4.6 s (tiptronic: 5.1s)
Elasticity (not sure what they mean with this ) 14.8 s (tip 17.0)