SP holders are not protected against the dilution caused by ballooning virtual supply (only from rewards, which are relatively small at this point). It does mean that SP dilution due to rewards is reduced, but the debt expansion as STEEM price declines is still very real.

In fact I'm not sure whether what you said about it being more attractive to power up now make sense at all. At 5% or 8% liquid the SP dilution is pretty low already. At 11% it might be slightly negative over time but you are risking another 5.5% instant dilution if SBD grows to 10% of market cap from its current 4.5%. Plus risking potentially severe price declines if SBD holders decide to dump rather than risk unlimited losses at >10% market cap.