I went on a 284 tear and looked at most of the neck downs, 6.5 included. If my notes are right 140 Berger VLDs and 139 Scenars went best, both with moly and on top of Reloader 22. H4350 was good as well.
I like 284 brass very much, you almost can't get a bad piece. It has good capacity for its length. What mostly knocked it out of contention for me was that it being slightly longer than 308 Win based cases put its OAL outside magazine confines with a lot of modern VLD bullets.
That said, I think 25-284 might be the best 0.257" cartridge going, at least until someone invents a really good 0.257" VLD. 6.5-284 might also be the best short action 6.5.