I've recently did a total rebuild on my dual 2.5's LCS and it's humming along nicely now.

Some questions :
1. Did you redo the thermal calibration after converting to air cooled? This has to be done with the LCS pump still plugged in.
2. Do you experience the same effect ( fans running high speed ) in OSX?
3. At what temperature does your CPU(s) run in OSX vs MorphOS?

One thing I noticed is that OSX is much better at reducing the frequency of the CPU(s) when not in use, thus the system generally runs a lot cooler than MorphOS ( My CPU's run at 45 and 48c in OSX but in MorphOS the main one is pegged at 71c)

What I've learned is that MorphOS always runs the CPU's at max frequency no matter what, thus they will run hotter all the time.

Hope this helps.