Using the same dyno? Same engine/head person? Same trans guy? MPH down? 60s? Incrementals? All of a sudden or over a period of time? Have you look at things like the rear, brakes, fuel volume, tried different fuels? Has the carb been looked at by someone? I know a guy had a fast E car till he started "tuning" the TQ and the car slowed significantly.

Agree with all the suggestions. Plug wires, ignition boxes, master switches, grounds, are all things that can cause issues without being obviously broken.