IMHO there are a couple of things that should be prioritised if you want to enhance the performance of the Stagea and build a base for any future mods. They are very simply:
Exhaust
Intake
Engine Management
The RB engines respond especially well to fitting a decent sized exhaust. On the Series 2, its 2.5" standard. I opted for a good 3" system with a split dump pipe and high flow cat. The engine revs smoother, and will maximise the addition of other mods.
The intake can be relatively cheaply improved too, with either a high flow filter (K&N etc) to fit the existing airbox, or a pod filter setup. These should assist with throttle response & economy.
Engine Management is another source of relatively easy to access power improvement. A piggy back system (like a Jaycar Independent Electronic Boost Controller - IEBC) can help you bring on boost earlier, resulting in more torque earlier in the rev range. Of course, this should be coupled with a Digital Fuel adjuster to ensure that the fuel/air ratios are optimal through the rev range.
Following (or before) that, consider a larger intercooler (FMIC are popular) to keep air cool to the plenum intake and increase horsepower.
The list of mods is really endless, but this should start you in the right direction. Hi Flowing the Turbo has been covered in other forum topics, but is of limited use (can be a bit laggy around town) if you havent at least upgraded your exhaust and intake.
Of course, everyone will have different paths that they have chosen, but this will provide you with a good base if you wish to crank out more power.
Hope that helps.