The iMac gives you 1440p no matter what because that's how many pixels it has. The difference is its either upscaled from 720p with the Belkin or 1080p with the Atlona.
OK I think the point of this is being missed...
yes the belkin supports 1080p or 1440p or whatever
yes the iMac screen res is 1440p
I already know all this
But once you connect the av360 into the display port and the hdmi from ur device into the av360, then the only resolution your going to get is 720p
The iMac only allows this and thats just it. Even though all devices connected can handle a much higher resolution, unfortunately you aint going to get it as thats just how it is.
Have a look at the provided link - http://www.belkin.com/IWCatProductPage.process?Product_Id=508267
Then click on features and read dot point number 4 which I will quote "Accepts full 1080p signal, provides 720p to iMac"