Sounds like you've got it sussed. Just a tune will make the car feel noticeably better, diff gears (probably more than exhaust work) and tune would be chalk and cheese.

- diff gears (assume you've got 3.46:1? change to 3.91:1)

- OTR intake is best, or the variety of cold air intakes on the market - note my memory fails me but do some research as the Monaro already has the preferable factory cold air set up. If like me your always on a budget, potentially better bang by putting money toward gears and exhaust

- exhaust ... generally 1 to 3 things you'll do: extractors / cats / cats back

- tune

Not sure who you're speaking to at Port Hacking, but these are guys I'd be comparing and seeking advice from:

Autowerks Automotive at Beverly Hills http://www.autowerksautomotive.com.au/

Sam’s Performance at Greenacre http://www.samsperformance.com.au/

Castle Hill Exhaust at Castle Hill http://www.castlehillexhaust.com.au/

(exhaust only) Sureflo Exhaust at Ingleburn http://www.surefloexhaust.com.au/

GM Motorsport's Ripshift are very well regarded, probably thought the best aftermarket shifter, but I havent had used one personally.

Other than a little update being required once the diff gears are done (i.e. speedo), do the other things you want before getting the tune.