It stands to reason. Give a pint of blood, knock out 12% of your oxygen transport capability. Go from 1000 feet to 4000 feet, you go from 20.1% oxygen to 17.9%. a thousand feet to ten thousand feet is taking you from 20% oxygen to 14%... that's a 30% hit. Probably not quite like giving up three pints of blood, but also not like shotgunning a Red Bull either. Had you driven there you would have noticed your car sucks, too. We knew from dyno tests that a normally-aspirated motor makes about 75-80% of the power it would at sea level. I built a full-roller V8 and drove it to school. It broke in about Utah and went from "satisfying" to "fun." Coming down out of Snoqualmie pass it went from "fun" to "oh holy shit I've built a monster." I probably gained 200 HP in 1500 miles.
Yup, those numbers look about right. Can't wait to run a 50k at 6,000ft...