I think what's slowing down the WP25 is actually probably slime build up in the magnet area.
My theory is that the tolerances aren't as tight as a Tunze so more brown slime build up can occur. I've noticed when I clean the pumps, I really need to clean the magnetic surfaces on the impeller and pump itself to get the performance back up to par. Using thick amounts of aquarium safe silicone grease in this area may prevent the slime build up but I'm not sure.
Does anyone know the size and flow difference between the 6055 and WP25? Since I have a full Apex anyway, I think I may eventually switch over to the Tunze pumps as it's a lot more economical if you already have an aquarium controller.