From the discrete designs (nevermind the monolithic designs) I've built and tested it might be higher than you think, at least that's what I've measured.
The transformer design I'm using on my preamp has a measure < 1 degree phase shift at 20Hz. THD+N is < .0005% at 1kHz. Response is varies by a maximum of 0.02dB at 20Hz. Again, all this is very dependent on the load, but since I control this parameter (a fixed input impedance, stepped attenuator follows the transformer), that's rather easy.
I'd be glad to compare this to some discrete designs of your choosing, but these numbers are pretty much down in the noise. They may be even better than this, I just cannot measure it.