I tried to look for the site a while ago but http://obd.wikispaces.com/ it seems it has gone down. Anyone know why?
My view on Haki is that it lets you read your opponents next attack, similar to how the sharingan does. So while nobody actually has lightspeed reactions, they can read attacks that move at lightspeed. Otherwise no one would be able to counter Kizaru and he would be the most powerful character in the series. It's similar to the Force in Star Wars, two haki users can read each others attacks, akin to pre-cog, which is why Kizaru doesn't stomp on everyone. The greater the control over haki, the more of an advantage a person has in a fight. That's also why someone like Shanks, who seems to have no DF, can fight on par with any of the Admirals or Whitebeard.
That's why in my opinion Haki gives Team OP a massive advantage because they have been shown to be able to read attacks that move at lightspeed, i.e. any attack by Kizaru or the Pacifista laser which is effectively a Kizaru laser.
Take away the haki and they lose, because like Girugamesh said, they have hypersonic+ reactions. They are possibly faster than the ninja's in Akatsuki, but people like Pein and Itachi have some hax techniques that would give them the win.