Killer- I really dont think there is anything to worry about here. You lost maybe 1mph at the most. That could just come from the extra 100lbs. Or any other factor. I wouldn't worry about any of this. Your car still runs great and I think you're going to be really happy once you get the new heads/cam in.
5400rpm is pretty high for stock heads. My car I get best results shifting at 5600. I think 5000 was probably a lot better.
Anyways, good luck, and I'm looking forward to the results after everything is done.
Sky
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-1989 Saleen Mustang #406- TFS Heads, E-303, edelbrock intake,70MM TB, 73mm MAF, off road H, headers and 3chamber flows.
12.55@107mph
Runs on 87 Octane and gets 24mpg!