I've been meaning to jump in to this thread...

I have an HP 425 with an 7980 attached running NetBSD 1.1 (yes really - I keep 
thinking about upgrading it but it never happens) that happily reads and 
writes tapes at both 1600 and 6250bpi.  In fact thats pretty much all it does 
these days and very little of that even.

cheers
mark