Daal, IB does pay credit interest on cash positions. Obviously there is a spread, so moving cash out of your base currency using IB's cash management will cost you a few percent a year of carry on top of whatever the interest rate differential is. However it won't be a 6-8% difference... nowhere near (unless you're borrowing dollars to buy yen or something like that). You can look up the debit and credit interest rates on IB's web site.
Anyway, the smart way to do this is with futures. That way you can hedge your dollar exposure to whatever currency you want with almost no cost of carry.
To be precise, here are the actual costs of hedging to another currency using futures:
1) The interest rate differential between the currencies (which could be positive or negative). This is built into the pricing of the futures contract.
2) The margin requirement of the futures position, which is probably around 5%. So you forego your cost of capital on 5% of the value of the contract. If you think you can earn 20% a year trading, hedging your entire account to another currency with futures will cost you 1% a year of foregone trading gains.
3) Commissions on buying, selling, and rolling over the futures contract.
Also you need to be comfortable with the size of the futures contract, usually $50k and up.
Martin