I don't think this is quite right. The rate limiting step during mashing is gelatinization (unpacking of the starch chains by surrounding them with water.) Gelatinization is the first step in the chain of reactions, since enzymes cannot act on ungelatinized starch. The enzymes are dissolved in the water/wort, and depend on the liquid for the mobility needed to come into proper contact and orientation with the starch chains, so that they can hydrolyze the alpha 1-4 bonds. As you state, the hydrolysis is fairly rapid once the starch chains are gelatinized. However, the molecules created by hydrolysis (mono and di saccharides [both fermentable], and more complex poly saccharides [non-fermentable]) are in solution when they are created. There is never any solid sugar, nor sugar trapped in a solid matrix, in the mash. There will be concentration gradients, with higher sugar concentration near the gelatinized surface layer of the grits (where the hydrolysis is taking place.) These gradients can easily by homogenized by recirculation or stirring. Homogenization will also occur by diffusion of sugars throughout the wort, but somewhat more slowly than with recirc or stirring.I think we need to distinguish between conversion and extraction. The starch can be converted quickly, but the sugars will still be entrained in the matrix of the solids remaining. It takes time, varying with your system, to fully leach the extract out into your wort. Conversion is quick and relatively consistent from system to system. Extraction, the key factor in efficiency, is quite variable. Terminology here is variable, but I'm sure you guys get my point.
You cannot get 100% conversion efficiency unless you achieve 100% gelatinization of all of the starch. 100% conversion efficiency means that everything in the malt that could possibly be dissolved in the wort has been dissolved in the wort. It does not guarantee that the wort has reached maximum fermentability, as you may still have soluble starch or large dextrins in the wort, if the enzymes haven't been given enough time to finish, or the enzymes have all been denatured before they could finish.
There is a widespread misconception that alpha amylase does not create fermentable sugars. Alpha amylase hydrolyzes (breaks) bonds at random locations along starch and polysaccharide chains, and will keep acting as long as there are alpha 1-4 bonds that the enzyme can reach (those not too close to an alpha 1-6 branching bond.) So, early in the saccharification process, alpha produces primarily long chain polysaccharides, which are unfermentable. However, as the process proceeds over time, the chains resulting from alpha amylase action keep getting shorter and shorter, until all that is left are fermentable sugars and limit dextrins (a branched polysaccaride with a few glucose units on each branch, which can no longer be acted on by alpha amylase.)
Evidence for the above is brewers adding alpha amylase to stuck fermentations in order to bring the FG lower. This works when the FG is higher than expected due to low wort fermentability. The alpha amylase acts on the unfermentable sugar to create additional fermentable sugar, which then allows the yeast to create more alcohol and lower the FG. The risk with adding alpha amylase to the fermenter, is that the amylase will not denature at fermentation temps, and the action will continue until nothing is left but alcohol and limit dextrins, leading to a lower FG than desired.
Brew on