This was displayed in a thread on the C7 forum discussing issues w Direct Injection and effect of not having fuel sprayed on valves.
The dyno chart demonstrates the gradual loss of power over time. I suspect something like this may also happen if you're not using ur secondaries very often especially since oil fumes are quite present in the LT-5 air intake.
The third and fourth run show difference in same motor after de-coking of valves. Fairly dramatic.