I base my extract recipes on extra-light DME just to avoid fermentability issues, it seems to ferment well enough.
I can see this working though, given the order of a typical step mash where you get a lot of simple sugars firt, then some larger stuff that adds body. Remashing at the lower temp with new malt is key since the beta-amylase in the original wort is destroyed by the higher temps favoring alpha-amylase.
And yes there will be some stuff that remains unfermentable but considering the real extract of a typical 75% apparent attenuation brew can have 35% unfermentables (Noonan's Lager book says 40% more than apparent), it might be that a good portion of those could be converted with the extra enzyme.
I've always thought the beta and alpha temp optima were backwards with respect to how I'd like them to be.