For a stock EFI 304ci, power up the best place to start is a thorough tune up.
Items required (in logical order).
1/ Oil change - with filter.
Go for a good quality oil. Perhaps a Nulon engine oil flushing agent is also recommended.
2/ Fuel filter.
This is located near the RHS rear axle & should be changed regularly to ensure good fuel supply.
3/ Spark plugs.
These need replacing fairly regularly in an EFI Holden. I'd recommend NGK BPR-6EF, gapped to around 0.95-1.0mm, or I have had recent succsess with the "non-resistor" NGK BP-6EF. I have heard this can interfere with some car sound/moble phone gear.
4/ Air filter.
Repalce this paper element. It is a cost effective upgrade to cotton panel filter that is cleanable. K&N is one that comes to mind - You may find a few extra horses as well.
5/ HT leads.
Although the std leads are fine, you will benifit from upgradeing to "spiral core" leads. Use at least 8mm thickness. If feeling generous go for 10mm, but these will only really benifit high HP cars. It's important to use the spacers well, and to keep them iscolated from the engine/chassis, which will minimise possible shorts.
6/ Distributor cap & rotor button replacement.
These items are a b***h to do, but the throttle response improvement is worth the pain if this has not been done for a while. Make sure you identify #1 cylinder, and connect the leads back in correct firing order, as to locate #1 cylinder is not a job for the amatuer.
7/ Ignition coil & ignition controle module.
Not an essential item, but is is worth replacing if this hasn't been done for a while. As it is in a bugger of a spot, perhaps re-locating it to the firewall is not a bad idea. The ignition control module can be a sorce of problems in the efi Holden V8. Sometimes these just go for no reason & give U a totally dead car. Often steam cleaning the engine bay can cause isses, so if U do this, perhaps cover the connections with a plastic bag or something to prevent this happening.
8/ Cooling system.
A poor cooling system will make the car run hot & rob valuable power. It is a very good idea to ensure you have a full cooling system, and no leaks. A cooler thermostat is not a bad upgrade either (say 170degF). Ensure the clutch fan is working, and free spinning once the engine is up to temperature. Lots of HP can be lost with a sticking clutch fan.
9/ Throttle cable adjustment.
There are lots of cars with very slack throttle adjustments. Often they can not open the throttle body fully in some cases. The cable adjustment is accesses by removing the 2 fuel rail covers (allen key), and removingthe wing nut from under the throttle body plate.
10/ Faulty earth points.
A fair amount of people often complain about unstable idle. After having this complaint also, it was discovered that a a short in the earth caused this. As the car relies on feedback from sensors to the computer, there is a high possiblily that either one of the sensors becomes faulty or the wiring earth is poor. One common one is the throttle position sensor (TPS), and the coolant temp sensor have the same earth. If both of these are not functioning correctly the car can think its under part throttle at idle etc causing it to run like a pig, use more fuel than std, and rob you of valuable HP. The main earth strap from the engine to the chassis also needs to be in good nick
Your car will love you forever, and you would have restored any lost power over time.
A compression check, and a base timing adjustment to 10deg advance are also things to do. Looking further, transmission & diff oil changes should not be overlooked either.
