It's already possible, look at Forza 5 and Strider, the question is more along the lines of what needs to be sacrificed visually in order to achieve that performance, they will remove all form of AA, refuce special effects quality (compared to a PS4 equivalent).
Keep in mind that AAA games will keep evolving and they will probably be more demanding than they are now in a few years if anything... When you see something like the Order 1886 in action, every big studio will now aim for that level of visual fidelity... which means that neither console will get constant 60fps on the big games, when the devs decide to push too far.
Yes tricks and API/SDK level optimisations will be made, same goes for Sony, the relative perfomance of each machines will always be more or less 50% (this only takes into account the easily measurable GPU differences, the memory bottlenecks in the XB1 only makes it worst than expected).
