I have the sew-steady/dream world extension table for my Viking & like it. The down side, as you mentioned, is that it's really not ergonomic because the machine & table are up higher than an built in model.
If you piece on one, then quilt on the other can you not just get an insert for the Janome for your current cabinet, piece your quilt, then switch the machine you use in the cabinet to quilt it? Or do you have multiple projects and are piecing & quilting, at the same time (but not simultaneously

)