Slower drives in my experience definitely produce less heat, probably less friction or same friction but less overal movement in the same time interval. Spin up times for fast drives is typically longer, comparing the spin up of a 4200 rpm drive to a 15000 rpm drive, the 4200 drive beats it hands down.

The only things that are improved with a high rpm drive are the time the head has to wait for the right data to appear after seeking to a new cylinder and possibly the max. transfer rate of the data. The wait after seek time is probably the predominant factor, especially on an empeg which doesn't use DMA, so higher transfer rates can't really make the actual IO happen any faster.

One thing that might help a lot more for the empeg is when the ext2 filesystem is modified to avoid scattering data across the various block groups on the disk. The access patterns (both write and read) are very much unlike typical UNIX accesses, and the filesystem isn't really optimized for it.