Minecraft – Are generic mob farms efficient on populated MP servers

The best would be to build it in stages.

• You start in one corner of the future, large spawner to make a small spawner only to craft TNT.
• Then you make a tunnel around the planned perimeter of the spawner while the spawner is producing.
• Then you use that TNT to start mining blocks on the opposite end of the spawner area and work your way back to the place you started.
• While your small spawner is producing TNT, you can work on the details of the spawner in the TNT mined areas.

In the end you will have the mob spawner of the size you need.

Replace the stone brick slabs with full blocks and make sure that your landing pads definately have 3-4 blocks of open space above them.

Also wither skeletons have specific light conditions requiring them to spawn the wiki page is linked below.

• Wither skeletons will only spawn near Nether fortress in light levels of 7 or less.
• Wither Skeletons can spawn in a 2.5 block tall space

http://minecraft.gamepedia.com/Wither_Skeleton#Spawning

Wither skeletons are a rare spawn so don't expect extremely high rates but the above should sort out your spawning issues.