I know that a very small percentage of flaked barley will raise a huge head. I'm guessing that only a portion of the original beta-glucans exist in Chit malt since the 'full' malting process converts most of them and this is partially malted grain.
I agree that flaked barley imparts a raw flavor that I don't like in pale beers, but its OK in roasty beers. Even at 1%, flaked barley can create a huge head and impart that flavor. I don't have that flavor impact when I use flaked wheat. I still get a decent head production, but the flavor is more wheaty and grainy. I like that better.
My findings are that you don't need much flaked barley to overdose your beer. In some respects, I'm surprised that brewers use as much Chit malt in their grist. But I suppose that Chit malt must be well on its way to being converted when its kilned and the amount of beta-glucan is actually very low in comparison to raw or flaked barley.