I'm not 100% sure on the fees but they are not even close to 0.3% for forex. It's pretty much around $2 for most trades (their example covers: USD 50,000 daily volume for EUR = USD 2.00). Here are the details of the fees: https://www.interactivebrokers.com/en/index.php?f=commission...
I think the reason IB charges are so low is because they deal with very large customers (banks, trading houses, etc.).