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Let's get back to basics here, I didn't read this entire post but is your crank pinned, your MPH is low, your et is what a stock c5 can pull off. We need to get back to basics, I wish you were in Ma/CT are.
If you are seeing 13 psi of boost, then you can rule out belts slip, crank slip, boost leak etc. 505 hp is a little low for 13 psi.
I suspect your problem is all in the tune. You may be getting knock which is pulling a lot of timing. You need to find a good tuner and access to a dyno with a wide band. Until then I would stay out of boost and definitely stay off the track.