Again, the evidence doesn't support that claim.
* In "Twisted", Janeway says to Harry that he has EXCEEDED all expectations. Not "doing the bare minimum" at all.
* Harry aided in the design and construction of the Delta Flyer, and aided Seven in constructing her astrometrics lab. Clearly, when something needs building, he's the go-to guy.
* There is flexibility in ranks: security can be run by either a lieutenant, or a lieutenant commander. And Tom remains at con despite being reduced to ensign. Ergo, Harry could certainly be at ops as a lieutenant.
* Harry's duties made him among the "senior officers" on Voyager. We don't see Lt. Ayala or Lt. Carey there. Clearly, Harry is punching above his weight here.
* If Troi, Geordi, Ogawa, Worf, Dax, Bashir, Sisko, Tuvok, and Tom can get promoted without changing jobs, so can Harry.