You might tell me different cuz you run the dyno, but 10HP on a 500HP engine is 2%. I'd consider that 'insignificant' as it probably is right around the accuracy level of the meters, sensors, and dyno itself. Right or wrong?
I'm not entirely sure what you are asking here. 10HP loss or gain is not insignificant as far as dyno accuracy goes--1-3 hp yes but more important is torque @ "the hit" tells me more about how the test is going to go than a 1-3hp loss/gain. 10hp on any dyno either way should be easy to gain back if you lost it, if you didn't make a whole pile of changes at once. I usually do 2 "pulls" at a time back to back and the data is very reliable.
All being told-without the electric water pump this is a solid 520-525 hp engine all day--I ran this pretty hard executing lots of warm up idle time, static load testing, and 15 rapid dyno pulls, oil pressure @ idle and 190 F oil temp was never below 40 psi, so I'm confident this engine is going to live a long and healthy life. J.Rob