The exact crosses are not entirely known but rumored to include Rhode Island Reds as well as White Leghorns (supposedly) and several other breeds. The egg production came from selective breeding. Simply being a Mediterranean breed does not mean you will have high production value, select breeding programs do. There is a big difference between a White Leghorn and a Brown leghorn in terms of eggs per year despite being the same 'breed' but different color and selection process.
The point is the egg production value of the Australorp isn't what it used to be because other breeds have filled the role of commercial egg layers eating less feed per egg. No longer used for commercial egg laying, the selection specific to that purpose isn't as strong instead focusing on the physical traits. I doubt you will find BAs with the production value they had in the 1920-1930 period.
But that's what I was getting at, you can easily find an Australorp today that lays 300+ eggs a year, any hatchery bird will do that, or come awfully close. The point I was making was they won't look like Australorps according to the Standard, which in the spirit of this thread is fine. I would bet a significant amount of money that the record setting birds didn't look like Standard Australorps either. My athlete analogy is a carefully considered one, an athlete can train to be a weight lifter or a distance runner, but not both. In the same way that a chicken flock can over time be bred for great meat or egg production, but not both. Dual purpose breeds were developed to bridge the gap, give a decent number of eggs and good meat. You can take any of the dual purpose breeds and breed them for extreme or the other, as was pointed out with the Sussex.
You simply cannot paint with a broad brush either. White "Leghorns" will out produce a Light Brown Leghorn, but that's simply because the "Leghorns" of today are actually hybrids that have been selectively bred for it commercially. If you were to take Leghorns regardless of color from different breeders and use the same selection criteria you would get similar results regardless of color, some of the more unusual less popular colors would take more work simply because of the numbers game. I have raised both White and Light Brown Leghorns (non-commercial) and after a couple years egg production was identical. Now working with Buff, and will be using the same selection criteria and they will produce the same.
That last paragraph is really true of any breed that doesn't come from a commercial source. Production aspects are what you make, which I guess is what this thread is all about. Taking what you have an elevating their production standards.