That’s quite an early SR8, first question is current spec?
Presumably 2.6 motor, and does it have “plenum” intake or 8 separate throttle bodies?
If it’s still on early plenum – there is your most cost effective and biggest hike in free revving and response, for 8 throttle bodies.
If it’s 2.6 on individual throttle bodies, you will be into diminishing small returns for any further tuning.
Unlocking ECU itself will give you adjustment but little or no power increase as it is already a balanced calibration.
Change to life racing ECU and loom would bring it up to date and give greater scope.
The standard Suzuki Hyabusa heads are very good gas flow as standard. We did further modify the ports and protruding valve guides, to carefully improve flow, so there are very limited gains to see on an otherwise standard motor. Increasing compression ratio with pistons could be a good step, as would increasing capacity, 2.8 litre with increase bore size, or 2.7 litre increased stroke (later 1340 base motor ) are two other directions for more power.
Today, I would go to RLM racing, who worked those Era of motors originally, know them very well with good historical advice, and modify probably more V8’s than anyone for increased power.
Hope this helps,