instant, I am going to disagree with you on this. Primarily because you haven't qualified what you mean by mbs have fallen since then.
The book value of the loans is primarily based on constant prepayment rate. As rates fall, CPR increases for the most part because people like me are calculating that refinancing my current not at x% less interest rate offset by the $1400 closing fees will pay for itself in 8 months... so someone like me refinances expecting to make the $1400 back in 8 months for example.
Rates have been low for some time and can't really get any lower, so CPR is going to be pretty strong (ie low). CPR might continue to get worse for another quarter as people who haven't yet decided to refinance finally make the plunge to do so. But for the most part my estimation is that CPR will remain strong.
A strong CPR means a strong book value. So i am not sure that book value will drop after March. Possibly the Q1 book value will drop, but there after I don't expect it to.