The problem with a .80/320 decent is that if the winds are not as forecast or ATC changes you speed or route (like cutting a corner) you will find yourself high and the jet will nose over to capture the path. At .80/320 you will find yourself out of Vnav path in short order struggling to get back on the path in order to make the crossing restriction.
I see no reason to plan a high speed decent. If you plan a .78/280 decent The Vnav will plan an earlier decent path. You can push up the power if you desire and fly .80/320.
We use a CI of anywhere from 12 to 99 depending on my dispatchers plan. I routinely override the cruise speed from ECON to LRC to get a higher cruise mach at lower CI'es. For example, with a CI of 12 a typical cruise mach is .76. If I manually select LRC the cruise will pick up to .79. I'm paid by the minute but don't like to spend one more minute at work than necessary.