You could leave 250K in the bank and move the excess to IB. You could split the money between several banks (preferably not more than 100K per bank). I do not know if the rules on the 250K limit have changed but I remember reading the rules applicable when the government first started the 250K FDIC insurance that had some qualifications ( the insurance would cover up to 250K but not to exceed the dollar amount in the account at the time the 250K insurance program started or 100K). These qualifications my no longer exist.
As for the safety of large accounts at IB, I think the large cash balances are safer at IB than any bank based on what I have seen happen with banks both large and small. I like the idea of dividing the cash among several brokers and clearing firms with IB being the safest self clearing broker to trust with cash. Just my opinion which of course is not always correct.
That is one great user name you came up with for use on ET.