I would say that you should try doing a thinner mash and add more water. I've had this problem as well with a very similar setup and have had to throttle down the pump a lot. Here are 2 more recommendations that may help. 1) Use a paint strainer bag from Home Depot, seems to let the liquid through a little better than voile bags do in my experience. 2) If using the basket along with a bag then try drilling the holes in the bottom a little bigger. I did this on the bottom of the basket and a few inches up on the sides and I believe it improves the flow. I like using the basket along with the bag because it prevents the bag from being sucked into the pump while recirculating. And also it is easier to lift out after mashing and allowing the wort to drain out of the grain.
Hope some of this helps, cheers!
Sent from my DROID RAZR using Home Brew mobile app