I am experiencing the same problem and just now successfully removed the impeller. Mine really did need to come off as the lower filter was badly clogged on the bottom side. If you have removed the screw, reinsert it all the way in, then back it out obout 3 full turn so there will be a gap between the underside of the screw and the impeller. Leave the 5/16" socket (without handle) in place on top of the screw. Next, I used 2 tiny prybars, (.5" x 5.5" and 3/4" x 7.5" but two of either size should work well) to lever beneath the impeller while being very careful not to push to hard on the pieces below. Fully insert the short end of the prybars under the impeller and place them to the sides at 180 degrees apart. While exerting a firm and balanced pressure on the prybars, have a second person tap squarely on the socket with a small hammer. The impeller should snap loose and move just a tiny bit upward to the screwhead. Back out the screw 2 or 3 additional full turns and repeat. I would not do this a third time as the few threads still making contact may be damaged. Now you should be able to remove the screw and manually pull off the impeller.
Disclaimer: Parts could be damaged or destroyed if excessive pressures are used on the prybars or with the hammer. This would best be attempted by a person with small machinery repair experience and a calibrated hammer.