the ONLY situation I've come across where filing separately makes FINANCIAL sense is if it relates to student loan repayments. (the spouse with the student loans doesn't make a lot of money while the spouse without student loans makes a lot of money ...... student loan payments will be lower if filing MFS!).
but as other have stated, once you file Joint and the filing deadline passes, you can't amend to MFS.
by chance did you file an extension PRIOR TO filing your Joint Tax return - in that case you could amend through Oct 15/
