I'm on CentOS, a Linux disto, not Windows.

Sorry. I remembered this reference to XP, but misremembered who made it.

I did issue the 'sync' command, but it apparently only commits buffer cache to disk rather than emptying it.

A crude but usually effective way of flushing one file from the cache is to cat a file that is bigger than the cache. Say, copy/append your 80GB datafile to another file 5 times (=400GB), and then cat that to /dev/null before running your tests. Might work for you.

I'm able to cat my testfile to /dev/null in under one second.

Assuming this is your 10e6 record testfile, and it is representative of your 80GB file and has an average of 86 characters/line, that gives a filesize of ~820MB.

The very best sequential-read throughput figure I can find for a non-raided 15k local drive is a little over 100MB/s.

That pretty much confirms that your testing is reading from cache rather than from disk. Even the most optimistic read-ahead algorithm cannot drive the interface 8 times faster than its maximum throughput.