One re-uses Diafine over and over - if you don't contaminate it, it will last for a very long time and will develop a lot of film.
With some films, it gives a real speed increase - e.g. old Tri-X at EI 1250 and you got good shadow detail. I haven't used it in years, so I don't know what EI people are getting with current Tri-X.
It accomplishes its speed increase by, inter alia, lowering contrast. So one needs to try it to determine whether it will be suitable for one's needs.
It might do a good job with negatives designed to be scanned - you should ask someone who uses it for that purpose.