Ozzyarb I suggest you to stick with the AUD base currency and borrow USD for your USD stock position. That way you're fully hedged against your local currency for the size of your position. You will be effectively holding X amount of USD based asset (stocks) and at the same time selling short the same X amount of USD dollar. As a bonus you're also paying a low interest rate (USD) and collecting higher interest rate (AUD). Of course if you convert to USD it could go up and you make money but that's a FX trade by itself.
Changing your base currency to USD and buying the same amount in AUD is *exactly* the same as sticking with your current AUD base account and you would be doing two unnecessary FX conversion which would cost your the spread. It is completely identical which base currency you chose. The only difference is the statement report and the process of wire in/out IB. Wiring in/out in USD can be pain in the a$$ for people living outside US because you will need to wire to the Citibank of New York where as with a local base currency account you would be wiring in/out your local Citibank in your own currency (no currency conversion needed, less wiring cost, and also much faster).
Changing your base currency to USD and buying the same amount in AUD is *exactly* the same as sticking with your current AUD base account and you would be doing two unnecessary FX conversion which would cost your the spread. It is completely identical which base currency you chose. The only difference is the statement report and the process of wire in/out IB. Wiring in/out in USD can be pain in the a$$ for people living outside US because you will need to wire to the Citibank of New York where as with a local base currency account you would be wiring in/out your local Citibank in your own currency (no currency conversion needed, less wiring cost, and also much faster).