One thing I've began considering is if it's worth stopping at the soft cap purely to allow higher tier upgrades that don't give haste. If I can get haste to the point where I can lose some on an upgrade and still stay about the soft cap I thought I'd still be good to go. As it was just said...increasing haste beyond 600 has VERY limited returns so I would think that if an upgrade increases everything from SP to mana to crit noticably but drops my haste, it would still be ok if I'm above the soft cap.
Obviously the more then better and we always want to maximize our HpS but I also don't want to pass up on boosting other stats through upgrades just because my haste would "only" be 650.