A 5 degree diff will definitely burn more wood, the unit short cycles too much to burn clean. A natural draft Classic will run a long time even with a 5 F diff. And the classic had no heat exchange area on the exhaust so it had to run choked down.
I've seen customers who switch from a flat bottom no grate unit to a grated unit sometimes have loading issues. They load the wood too tight on the grate or let ash build up and either way it chokes. Then it rolls thick smoke and the efficiency drops out the bottom.
I'd open up the draft to get a clean burn (that unit needs all the 1TDR3 fan can put out), 15 F diff, clean the grate off (if needed), load the wood so air can blow by the front and mix and burn off gasses. You'd be amazed how much difference some experimenting makes.
Just some thoughts.