I have not experienced any change in fermentability by sparging with hotter water. It's a good theory, but I haven't seen it in practice.
Denny must be right on this. It runs counter to the concept of lower mash temp getting more fermentables, generally, but it has been my experience, as well.Don't confuse getting more extract with getting a more fermentable wort. Higher mash temperatures will often yield more sugars sooner (both fermentable and nonfermentable, potentially yielding a higher OG) than lower mash temperatures, but the cooler mash will usually be more fermentable (it will finish at a lower gravity from the same OG).
Nor have I, though I imagine it would depend on how much conversion has occurred before you raise the temperature. For me, there is little to no conversion left to occur during the sparge, so it can't change fermentability. I suppose that could be true for you, as well.
But anyway, that wasn't the statement I was trying to clarify. "lower mash temp getting more fermentables" was what I found misleading. A lower mash temperature can get you a more fermentable wort than a higher mash temperature (independent of the sparge), but a hotter step or sparge can increase the total extract by improving conversion. I suppose that might
decrease fermentability, but it is almost certain to increase the total amount of fermentables
in an under-converted mash, simply by improving conversion after a cooler mash.