Although there seems to be some Killer-hate around these parts, I have used both my Intel NIC and the Killer without any noticable difference in speed, latency or anything else (I am on a 50mb/s down/15mb/s up connection with avg 8-9ms pings)
It seems to me that the issues from earlier Killer NIC's have been resolved with the latest versions (e2400), when using up to date drivers and BIOS....
I haven't actually tested either since I got the board, but I can do so & post back this weekend.
And yes, your ISP controls your actual speeds, but with a good, solid NIC, at least you can make full use of whatever they give you to work with......