Once "powered up" the corresponding amount of SBD effectively "does not exist" any more, from that moment on it is just an extra line of code to calculate vesting_shares.
Therefore it shouldn't be taken into account for calculating the debt ratio.
If the amount of the liquid SBD is below the haircut line you could power down SBD as well.
Otherways you power down SBD to Steem taking in account the haircut.