I have two of those spindles, clamped to a board for testing. On one of them, the bearings are noticeably tighter than the other. But they seem to run at similar temperatures.
What I noticed, was the first time I ran them, they got pretty warm. But they seem to run cooler as they were ran more. The longest I have run them was maybe 20 minutes, and they probably didn't get much more than 100-110°F. But not running at 24,000.
What was the ambient temp when you were running them? It was about 60-65°F in my garage when I was running mine.
In practice, I will probably rarely run them over 18,000 rpm. And mostly in the 12,000-15,000 range. From what I've seen, I didn't think they would run that hot, but that perception may change when they are actually cutting.