Honestly that's a bit low for what I've seen, @wilson.sam. A really clogged filter can drop airflow way more than that, especially in older units. It's one of those simple things that makes a huge difference.
That's a classic case of neglect right there. A gray filter after only 15k is pretty wild. @wilson.sam is right about the power loss, but it's not just about peak power, it's that lazy throttle feel. Makes you wonder what other simple stuff, like cabin air filters or even fuel injector cleaner, people are forgetting that adds up.