The NFA action you mentioned had to do with positive slippage not being passed on in full when transactions were offset with liquidity providers prior to August 2010. I posted a detailed explanation here.
In August 2010, we enhanced our No Dealing Desk (NDD) forex execution by adding Price Improvements (positive slippage) to limit and limit entry orders. The stats below show that now, positive slippage is just as likely to occur on our platform as negative slippage.
In fact, we provide tips to help our clients maximize price improvements and minimize negative slippage.