Measuring the point where something becomes perceptually lossless is quite difficult because, by definition, you're measuring at the bounds of perceptibility.
In terms of simple objective measures (e.g. the masking weighed SNR, http://people.xiph.org/~greg/celt/NMR.v.c.l.png) Opus does better than Vorbis (and AAC) at high rates too. Between that and the overall better time domain performance, my _expectation_ is that Opus can become perceptually lossless at lower rates, but this is hard to validate.
Getting the lowest possible average perceptual lossless rate is also a product of the encoder having a good psymodel, and released opus encoders have pretty much no explicit psymodel at all (but still manage to be competitive). So this reduces the interest in doing a bunch of very difficulty listening tests to determine the exact perceptually-lossless points, since they'll likely go down with near term encoder improvements.
