Zhoutong, wouldn't it help your liquidity if you could increase order & trade frequency? I'm sure that would be a natural result if you could find a way to decrease the spread some. I know I'd be able to trade more - a $0.10 - $0.20 spread inside a $0.12 - $0.15 trading range doesn't leave much room to trade (unless you're trading huge, probably over-leveraged positions)
I understand that with as much leverage as you extend your customers, you have to protect yourself from significant swings, too.
I also have to wonder if maybe you ought to raise the requirements for BTC held in the currency account - severe fluctuations in the spot price of BTC/USD send heavily leveraged accounts holding (mostly) BTC spiraling down. It is often not wise to leverage the purchase of a risky asset with more of that same asset.
I have to admit, I cussed your pricing bot the other day, too - slippage was IMO, pretty steep. But I looked back through the books, and slippage the other direction was just as steep - a dead even wash from how I see it. I'm sure it could use some improvements, but that won't help with the fundamental issue of BTC holders being overleveraged.