The stuff outside the OU cannot be seen in principle. Its size is fully independent of our current level of technology, and no advancement of technology will let you see beyond it. (Seeing beyond it would mean you can send signals faster than light, which implies you can build a time machine, in which case all bets are off.) In practice, we cannot see anywhere near as far as the radius of the OU, since after a certain distance you're looking so far into the past that space is opaque to microwave radiation. If you can somehow see neutrinos you get to see more, but that's still all inside the boundary of the OU.
Yes, I know that they cannot move faster than the speed of the light, and that the universe was opaque to visible light before the decoupling of radiation.
But there're a group of neutrinos that decoupled before the decoupling of radiation, even before this gravitational waves were at large in the universe, they could in principle say something about regions that are outside the Observable Universe, this ignoring the fact that they must be absolutely difficult to observe and that both neutrinos and gravitation waves are generated by new events and that we can in fact extract some information about these regions from them in the same way we can from light.
This was what I found from a paper, "Detection of gravitational waves with resonant antennas" from Francesco Ronga, earlier today:
"Gravitational wave and neutrino astronomy will increase the amount of observable universe, because they will investigate places that are completely inaccessible to the electromagnetic radiation and probably will change our knowledge of the universe evolution"
I think you're confusing two very different points in the history of the Universe. The CMBR was emitted after the decoupling of matter and radiation, as you mentioned, but this event was over 300,000 years after the Big Bang. And you are correct that there could, in principle, be observable signals emitted before this event in the form of neutrinos or gravitational waves. The observable universe, however, is defined not by reference to the signals emitted at the decoupling event, but rather the start of the inflationary epoch, which was merely 10^-36 seconds after the Big Bang. So, the OU is big enough that it encompasses all of the space from which any signal, even neutrino or gravitational, that might have been emitted 13.75 billion years earlier that could eventually reach us.
But, terminology aside, this still leaves your question: How much farther could we see if we could pick up neutrinos or gravity waves? Not a whole lot, unfortunately. Most of the expansion of the early Universe occurred during the (aptly named) inflationary epoch, and that lasted less than a tiny fraction of a second, leaving very little time for a neutrino or gravitational wave to travel before the Universe became very large. The expansion that occured in the following 300,000 years is negligible by comparison to that first tiny moment, so you won't get a whole lot more than 300,000 light years out of those neutrinos. I'm too tired and lazy to do the math to find the radius of the surface of last scattering, so I'll run a quick Google search, and...
The final answer is that the visible universe, or that which is not obscured by the opaque matter that dominated in the first 300K years, is a sphere with radius of 45.35 billion light years, while the observable universe, which is everything that can observed in principle, is just a bit further at a radius of 46.5 billion light years. So, yeah, we've got most of it covered.
