>And ISA is limited to something like 8MB/s, isn't it?

Well.. 8Mhz * 16-bits == 16MB/sec, if it could clock data every cycle.
In practice, the limit is about half that number, so you're right on.

But the current drive-to-drive copies are probably not even close to that.
Turn on IDE multmode and they'll get a LOT faster. I'm going to include multmode=8 as a standard setting in my next hijack kernels, as an experiment (oh ye guinea pigs..)..