I did an experiment:

A Quadra 610 running  a current system of 10 April 1997, running X.

A PB5200 running MacOS 7.5.3-DK with Open Transport.

The Quadra was turned off and cold-booted. The network seems to work.

The network is connected to two more computers, but they are idle.

I use Fetch 3.0.1 on the PB to upload a couple of reasonably large files
to the Quadra. 

Fetch logged the following transfer rates:

a 2 MB file: 134091 bytes/sec
emacs-19.35b.tar.gz (> 10MB): 147654 bytes/sec
xc-1.tar.gz (> 14MB): 150307 bytes/sec

To me this says that when the Sonic driver works at all, it works
quite well !

Running top on the Quadra during the upload says that the CPU
load is about 2-3 % to ftp, just below 60% system usage and still
about 40% idle time.

I send this message to say "Good work !" to the people who have been
developing and integrating the Sonic driver -- I'm sure that you'll
get the warm-boot-initialization problems ironed out, and then it looks
like we're going to have a nice driver.

- Erik Bertelsen