In addition to my observation (posted by oehhar on my behalf, thanks Harald) that I get full channel performance with plain TCL sockets on Windows 10, iocpsock in zero-byte receive mode maxes out at about 44MByte/s, reason unknown.  I had no time to investigate further since plain TCL works just fine...

I'm using a patched TCL which allows up to 10MB as -buffering for any channel (instead of the default 1MB), but that was also the case before on Windows-7 when plain TCL did not work and iocpsock did.