Australian Car Repair Issues? Ask an Auto Mechanic for Answers ASAP
I work for Holden/Opel, so I'll suggest a few things.
Your engine is a C22SE or C22SEL or Y22SE?
The timing belt is very important on your engine, how old is the belt?
I have seen timing belts fitted and not timed correctly, gives low power at low engine speed.
I have done many timing belts that transform the car back to near original performance.
The other part of the equation is carbon buildup in the cylinder head and injector flow/spray pattern.
We use a combustion chamber cleaning product (Subaru makes a good product) that cleans the majority of the carbon out.
Hope this helps.