Not correct at all. Tax rates don't get adjusted. Aus taxes them at their foreign-income tax rates (30% on first $90k, 37% from $90-$127k, 45% on anything > $127k, all converted to usd). The fighter CAN take advantage of a foreign tax credit but the problem is if you get pay more tax in a foreign country than you would have in the states, you do NOT get a refund. It's essentially lost because it's doubtful you're going to fight in a country in the same year with a lower tax rate than the US (I guess Saudi or something. In that situation your US taxes would be higher than Saudi, so any excess FTC credits would reduce that tax burden). But you cannot even use this "excess" taxation to offset taxes you would owe if you fought again the same year in the US, only foreign income.
Fighting in Australia is pants on head retarded as an American. If a fighter gets $300k, they would owe $75k to the US versus $119k to Aus. FTC says you paid $119k elsewhere, don't worry about the $75k but you're also receiving no benefit for the excess $45k you paid to Aus.