Like everything in the world of ponds, it all depends. Although you are correct it is nice to have the 55 degree well water as an option to inject cold water into the pond, then the other side of the equation is this. I'm injecting water that has very little oxygen in it directly to the part of the pond where the water might be a little less likely to exposed to the air. If I can continue to take that cold and low oxygen well water and mix it up to the top to get exposed to the air and pick up some oxygen that might be OK, and then we have to get it all the way back to the bottom without warming it up too much so that the benefit becomes a new supply of cold water that is carrying all the oxygen it can. Cold water carries more dissolved oxygen but it has to get the oxygen dissolved first!
Some can run the well water over some rocks or a waterfall to pre-oxygenate but my discharge line is buried under the frost line from the house out to the pond and comes out straight into the pond at about 4-5' of depth and about 30' out from shore. I notice when the plume from that outflow is going that it doesn't stay injected at depth, there is a little disturbance in the water at the surface too so some of that water must be going straight out, some up as well.
Adding some blue dye to the inflowing water and watching how it distributes would be neat.
Having a DO meter would also be nice
