On slippage I used 2waymarkets some years ago and I got slippage on my stops, I started if the price went past my stop I moved it slightly wider before they could close it and what happened the trade turned and went my way, but then all of a sudden my stops were closed dead on what I put and slippage disappeared. Recently with using ETX I had a ftse trade on and the market was moving fast, quiet slowly but the price went 3 points over my stop, I complained and they said slippage, I stopped using them, they were taking the p in a big way. I think if they are told I am not using you because of your slippage they would start to get the message. In all my years of using SB's they are the only time I have had slippage and that includes very fast markets as well, its only when the markets gaps I would be caught out but luckly I have never been on a wrong side of trade when a gap has happened.
I used to use CMC many years ago under their old name and I would never use them again no matter how good they became, leopards never change their spots.