The change in fermentability w/ change in mash temp is highly dependent on your base malt.
IME I haven't been able to significantly reduce fermentability (via high mash temp) on beers made with an American 2-row base. The DP is off the charts. Its bread to produce extremely fermentable worts, even with rice and corn.
Moral of the story - for various grists, don't expect the same change in fermentability with the same change in mash temp. Don't just select a "Mash Profile" in Beer Smith and move on - dive into the mash procedure, make sure you're temps, thickness, volumes, etc. are consistent. I don't even use the "Mash Profile" option for this reason.
Well maybe this is the route to explore. I've assumed that the increase in mouthfeel/viscosity/whatever of the old world grains have been because of more beta-glucans, but I wonder if this is a contributor as well.
What base malts would be less fermentable producing chewier beers?
Grain is my 'final frontier' in that I think I have a fairly good handle on the other ingredients and processes of brewing, but not grain. Can't wait for the Grain book in the Water, Yeast, Hops series.
Lesser-modified base malts can allow for a greater change in fermentability w/ mash temp: Maris Otter, Munich/Vienna, and floor-malted products. Plus they taste better in 'chewy' beers.
You can also add a touch of oats or wheat (less than 5%) to boost mouthfeel. I've gone away with the flaked stuff from the homebrew shop because I think it has a plasticy taste. I like using steel-cut oats - you just have to boil them first (make oatmeal).
With some homebrew systems, it can be a challenge to maintain the mash/runoff temp all the way through to the start of the boil. I have this issue: I run off slow, batch sparge, and only have one burner, so my first runnings temp can drop and hang out in the low 150's if I'm unorganized or not paying attention.