I've always batch sparged and have had 3 different coolers for mash tuns. Each one was setup different than any other and I've never been able to open the valve all the way without causing problems for myself. I always read Denny's instructions as "open the valve up as far as you can and let it run off". In my case that's between 50% and 75% open.
As for OP, it's most likely the flaked grains gumming things up. I seem to have slower run offs every time I bump up the flaked grains. Good warm water additions and taking my time gets the job done, usually.