I'm not loading up much rifle on my 550, but my powder measure has been checked again and again, and I'm not having any of the problems you report with the Dillon measure. In my experience the Dillon PM seems to be right on the money.
One or 2 key words in the posts above keep striking my brain and making me wonder about the powder measure setup you guys are describing. Words like "properly tightened" and "tightened down" strike me as
exactly opposite of what you really might want to get good, accurate drops. Tighten the 2 clamp/mounting screws 'yes'; secure the body of the hopper absolutely 'no'.
To get consistent powder dumps on any volumetric powder measure you need consistent
density. Therefore you want a consistent
vibration to shake the measure to a consistent density. To do this on my late model bell crank Dillon PM, I use the old-style Dillon PM return spring (as you can see in the photo) slackened off just a tad
to make the slide snap back. The snapping action is consistent due to the release cam on the bell crank. At that point the hopper gets a goodly jolt, thus leaving my "powder drops" right on the money. With this slight mod, I'd rate the Dillon PM right up there with the RCBS UniFlow or Redding, both of which I also own.
So your Dillon PM issue might resolve itself A) if the hopper is free to vibrate, and B) has the spring added to sling-shot the slide back home. And that's got to be $200 - $600 cheaper than any option mentioned above.
Hope this helps.