5 man raids are essentially capped with respect to difficulty. There's only so much you can do in a 2 minute fight. At the beginning of each expansion, healers have bad mana efficiency, which removes the possibility of 5-10 minute fights, unless you give the bosses all 1 shot mechanics and very little damage otherwise.
Also, if you think good guilds/groups of players figure out and kill HC raids fast now? The spread of skill is even higher in 5 man groups. If you tuned 5 man raids for a group, composed of, say, members of Sco's challenge mode group (look up streams of his early MoP CM gold medals), and they were supposed to wipe 100 times per boss? Then no one would down the bosses, ever. But if you tune them even slightly under, then people complain because method's best 5 cleared the raid that was supposed to last 3 months in 2 hours.
Plus, wouldn't the mechanics get really dull? You can't have the staples like tank switches, random mind controls, random disables, etc. because if the healer is targeted and the group is taking raid like damage, then there's no chance of beating it.
I love the idea of challenging 5 man content, but trying to integrate it into raiding would be a disaster of epic proportions.