>    Start reading from mode 4 drive - iostat shows 25 MB/s
>    Start reading from mode 2 drive - mode 4 drive slows down, so both
>    are at 9 MB/sec
>    Finish reading from mode 2 drive - mode 4 drive goes back to 25 MB/sec.

Here is a crazy suggestion that might help somebody work around that 
quirk.  Assume the disk has a read-ahead buffer.  If the data is 
in the buffer, a read will be fast.  If it's not, the read will take 
a while for the arm to get to the right cylinder and the head to 
rotate over the desired bits. 

So time the read.  If it was fast, process the next read on the same 
drive if there is one queued up.  If it was slow, switch to the other 
drive, if there is work queued up there. 

That's not the optimal way to do things, but it might be simple to 
implement and get a lot of performance improvement.