The 82plus program themselves maintain a list
of the efficiency characteristics of every supply they've tested. They don't generally record lower than 20% efficiency, unfortunately, but they do provide a graph of efficiency vs output, so you could extend the curve to a lower level to see an approximate value. If you're choosing a PSU with an appropriate capacity for your build, that should be close enough to give you the information you're looking for.
In general, switching from an 80plus supply to a PicoPSU will only save you a small single-digit number of watts; probably around 5% unless you're comparing to a very high-capacity supply. The main advantages to a Pico are in its size and keeping most of the waste heat outside the case. If you're upgrading from a non-80plus unit and the Pico appeals to you, it's a reasonable investment.
Finally, I'm not sure how close you'll be able to come to your performance goal at 80 watts. Meeting that with a fast dual-core processor is easy enough, but if you really want that 3D graphics performance in there, you're going to be looking at around 100 watts DC in any configuration I could come up with. Even with a lowly E5200 and an elderly HD 2600 Pro, you're probably looking at more than 80 watts (and I have a hard time recommending a card so old; you'd really want an HD 4670, which uses 20 watts more - there's not much choice in between those cards for power consumption, but they're in different leagues for performance.)