In this case, what I would do is make the area that requires a lot of loop cuts into a second object. I tried to avoid doing that at first, but there's no reason to. The computer is calculating face and vertex data. By having the extended part of the mesh as a separate object, you reduce the vertices in the first object, because the loop cuts can be removed altogether.
Select the extruded area of your mesh, separate it (I changed all my shortcuts so I have no idea what the default one is), and dissolve all of the old surplus edges in the other object.
It will stay low poly, and is just as easy to unwrap and manipulate. Just parent it to the old object if you'd like it to move at the same time, or group it, or whatever. Lots of options in this case.
Separate objects are good. Try downloading a couple of models from the Models Resource, you'll be amazed how often things are separated into multiple objects when they seem like they must be a single object.