Recently bought a 1997 XJ8 4.0lt and had nothing but probs.

Please can anyone help with the following:

1---oil useage is high, about 2lts per 1k
2----on hard accleraction there smoke form exhaust for a few seconds
3----Recently had a new gearbox replaced but the performance is poor.
upon checking the exhaust, the 2 stage box has a crack on the area where the pipe enters the silencer. Would this empact the box performance. I think the silencer is rotten inside. The exhaust is a Jag original, so dont know when it was last replaced.