How can I reduce the Tuple Mover's disk I/O impact for mergeouts?
What you need to do is make sense of what is your usual load size and set the size of your TM resource pool accordingly.
  Changing the  ActivePartitions to 2 will help you because will stop looking for older partitions to Consolidate. 

Answer me this questions : 


What is the size of your TM pool ?
How do you do your Trickle loads ? (they go to WOS or ROS)
How many partitions do you have in the table ?
How many projections do you have for that table ? (do you use them
all)
Do you run delete on this data ?(check for replay delete)
Your projection have the default sort ? (all columns or specific
columns ?)


What is the value of MaxMrgOutROSSizeMB and MoveOutSizePct,PurgeMergeoutPercent. 

Better off can you post the definition of your TM ?