I also assume the price (ratio) of Steem/SP will be < 1, because it provides immediate liquidity and having it fluctuate over time allows to "trade vested stake" as well!

However, because if Steem/SP < 1, that means it is cheaper to buy SP from the internal market than to power-up liquid Steem, hence everybody would then acquire their SP at the internal market, provided any tradeable SP supply is available.