I believe it's trading at a discount because of the 1-week delay in price feeds. If STEEM were increasing in value, SBD would be trading at a premium. I think... I haven't completely worked out all the details yet.
Anyway, the conversion price lags the real-world price by about half a week, which means I'm not confident that I'd make money by buying SBD at a discount and then converting it to STEEM.
@biophil Excellent insight and you are 100% spot on. People are pricing in the risk of a slide in the dollar value of steem. They want out and they want out now. Back when the price of steem was rising, you couldn't buy SBD for less than a 30% premium. I have a post from back when that was a fact, explaining to people that they should sell SBD on polo to drive the price back down to normal.
Right now, the very best thing you can do financially is either to hold SBD (which I've just found gains 10% interest), or convert it to steem thereby giving you about a 20% gain in dollar terms and also destroys the excess steem in the system.
Yeah, I'm running a SBD-to-STEEM conversion experiment right now where I'm converting a set amount of SBD per day and then re-purchasing SBD when the conversion goes through. My first attempt that I completed today netted me a little over 2% (I started with 200 SBD, ended up with just over 204). That is very very far from the 15% that all of the face values are suggesting, and I don't know where you came up with 20% - that's simply not available today. :)