I think the collateral and a 10% APR should be enough to hold the peg, the reason you see a price below 1 dollar now is because of a downtrend that keeps the exchanges price below the 7 day average shown on steemd, if or when price of steem stabilizes or increaes then you will see SBD at 1 dollar or above... I personally dont think we should assume that the SBD will be 1 exact dollar always, it will most likely vary from 0.80 to 1.20...
SBD has been alive for exactly 1 month today.. it has been below 1 dollar and it has been well above it too, so in my opinion is too soon to be thinking of changing the interest rate.
Exactly right that it will fluctuate. With the current "slow" incentives it appears SBD will be overvalued in a bull market, and undervalued in a bear one. It's just reactionary and slow to change the peg, which is why it would be interesting to have faster incentives to keep the peg as well!