Because the SU-27 would probably win, based soley on numbers.
Sorry, Huggy. While I love my platform, I'm also a realist. I know the biggest advantage I've got if I find myself in a merge with a Flanker, is that I've probably got a lot more BFM experience than that guy.
That is the problem with guys like physicists who talk about such things solely in numbers.
I think everyone here can agree that it is the pilot and not the plane that is going to win the battle. All of the countries that Australia might face in combat and fly Flankers have much less well trained pilots than the Aussies, and would likely get their a$$es handed to them in a fight, Flanker or not. One of the funniest reports I read was about one of the aforementioned countires and their pilots.
Anothher thing to consider is the maintenance of the aircraft. We like to complain about our planes but when you find out the reality about other countries (ie. not our allies) and their aircraft reliabilty, sortie rate and just how many they have that can actually fly, I wonder why I ever complained about our maintenance (that is, until the next '1 hour fix' from maintenance takes 6 hours......

). And one thing that is widely known is that the Russians are the worst when it comes to supplying parts and knowledge about maintaining their aircraft. These aircraft are usually supplied to thrid world countries where being a fighter pilot is prestigous and attracts 'quality' personnel, but their maintenance personnel are treated like dirt and have only a fraction of the training that ours do. If you have any experience with a third-world country and their enlisted folks, you will know what I mean.
An exception to this rule is India, and they only have so much sucess with Russian equipment, just check out their MiG-21 crash and reliabilty rate:
http://www.hinduonnet.com/fline/fl1911/19110550.htm
http://www.india-defence.com/reports/2367
So, I think Australia would be more than fine with either the JSF or the Super Hornet (only two squadrons would replace just the F-111). Especially against a country that has 20 Flankers (but really only 6 flyable, three radars working between them, with only 7 trained pilots, only two of which have fired actual weapons in practice, on average who have only flown 3 times at night or in IMC in the last year and have 34 hours total on average in the past 8 months).
But let me tell you about what those Flankers
could do........boy they sure look pretty on the flight line there........