Yeah, if the price feed tracked the real-world price perfectly, there would be no incentive to convert at $1. You're probably right that it wouldn't actually take much to push SBD up to $1, but I don't know this offhand. You could get a lower bound on how much Steem it would take by looking at all 3 order books: steem internal, bittrex SBD/BTC, and poloniex SBD/BTC. It's only a lower bound, because people have reserves that aren't currently sitting on the books.

One massive problem is the mismatch between the feed price and the real-world price when Steem is changing in value rapidly. Last week I put in conversion requests for a total of about 500 SBD. But because real-world Steem dropped so fast in the past couple days, the STEEM I got from the conversions is now worth less than $400. So I'm more than 20% in the hole on what should have been a sure bet, and that's going to dampen my enthusiasm to support the peg in the future.