Have a question regarding hard acceleration in first on my MY99 Turbo. If I change up at 5-5500 RPM it is very unpleasant, there seems to be a bit of dive and a thump sound.

Could there be a knackered engine mount or tired suspension (car has done 57000 miles), don't think it should be so harsh coming off the power.

 

TBH that could be several things, from worn bushes, to exhaust movement when the torque is removed off the transmission the engine moves back 'violently' causing all manner of movement.

I would have all the underside looked at by an Impreza specialist, or do a good visual yourself. Start at the rear diff and ckeck all fixtures to the shell, there are 8 fixing points. Then check the suspension for leaks/looseness etc. If the car has had a hard life due to the local roads, things could be tired under there.