A couple years ago I was informed by someone who worked in the industry that some s/b were trading against their clients, especially those who were doing well. So what you said could well be true.
I took a look at their trading system a few years ago, and did not like what I see, there was clearly a price delay and distortion then. So I went to trade the Fx spot and index futures instead.
Sorry that I can't help you. I'm not sure how easy it is to find other people to come out to support you with simliar experiences, for one thing, some people maybe just too embarrassed to admit it.
If you have concrete evidence, then why not consult a lawyer. Getting a class action can take a long time. One thing to bear in mind is that even you have solid proof that they delayed the trade, you'll have to prove CMC did it intentionaly. This can only be verified by using CMC's internal timestamps at the order receiving time and order dealt time. The tricky part is that if you use internet to connect to CMC server, then they can blame all the delay on internet.
take care,