only way to really tell man is if you want to dish out 50-75 dollars every year to do a quick dyno run just to see how much power your losing,
but even a dyno result wont be very accurate, like my car will keep pulling and gaining more HP after every run until it starts to really heat up then drops and drops until im right back where i started which in all was a good 10 runs to reach max WHP and another 4 to see the same results from the very first run,
to many conditions man but 15 whp loss in 9 years could or could not happen, their are people who will blow an engine in 6 months man just ask my cousin.