What does a mashout do in BIAB?
You raise the temperature to over the operating temperature of the enzymes and possibly even denaturing them (I haven't looked to see at what temperature the are denatured...) so as to effectively stop them working thereby holding the balance of maltose to longer chain unfermentible sugars in place while you faff around draining your bag and collecting additional runnings and whatnot.
Seems like a good idea to me. It's not going to improve efficiency (unless I'm missing something), the reverse if anything - it's going to stop the beta amylase during the rise quicker than the alpha so your efficiency is set then, you might get some more soluble but unfermentible sugar during the rise then all the enzymatic activity gets stopped...
...or at least *I think* that's why you do it. Surely it's more to do with the style of beer you want rather than the style of brewing you employ?