I'd like to bump this to get some advice as well, as I in effect have the same question.
I have 30k a month in available Bluebird spots, and I just passed a FR. Would I be safe to go for 10k a month to get the bonus? 1.5 MR per dollar is better than 1.07 UR per dollar by my calculation, and I'd have to go strong to get it.
I've read conflicting reports that you are "safe" for 6 months after a FR as long as you have the credit.
For what it's worth, I don't particularly "value" my relationship with AMEX, but I don't want to do anything too stupid - but I'm not above being aggressive, either. A 45k end of year fiesta would be a good use of value.