The ECU you are running is programmed to alert you to knock. This is a non-stock feature and was added by Mercuric. The CEL will toggle when the cumulative amount of ignition retard due to knock surpasses a set value. The value is 15º unless you changed the value in the tune before flashing. This doesn't necessarily mean that the ECU is currently detecting knock, but it does mean that the ECU detected knock in that general RPM and load range in the past.
Knock is the #1 killer of engines. Your engine could be knocking due to lean fueling, overly advanced ignition timing, or too much boost given your mods. Just because your tailpipe is black doesn't mean you're running rich under load, the only way to know if you're running the correct AFR is to install a wideband gauge. The CEL not triggering when the temperature implies that the problem is related to too much boost and/or ignition timing.
Improper spark plugs won't detect as knock, that isn't your problem.
@tighe it's VAST tuning now 😉