Thanks Hurricane, you all helped me a lot, because IB's service just sucks I'm used to excellent TOS customer service, looking to opening another account with IB though
Yes, you can always change your base currency (in account management). Probably it will even be applied immediately (aside from some interest and reporting it has no fundamental meaning). As said above, the 1,6% is only charged when holding overnight. For the long euro amount, IB doesn't pay interest for the first 80.000, but you can buy euro equity and sell the future on the equity to receive the interest (currently not much, but for a normal 4% rate this is useful).