It's pointless to talk about that 8 minute warning because the speed of light is effectively the speed of causality. There is no way we can signal back faster than that initial wave of radiation hitting us. There is no way to alert us that that 8 minutes is starting because are theoretical fastest communication will still take 8 minutes to get to us.
But we're not worried about that part of things anyway - it's the mass part of the CME that is the issue.
But we're not worried about that part of things anyway - it's the mass part of the CME that is the issue.