So for step mashing at both ranges with a protiene mash you get great results. Also keep in mind that BIAB is less efficient than a mash tun. You would actually get a very thing body beer if you did it the other way around but because you do 148 and 162 you get this:
1) the beta-amylase breaks down the long chains into shorter ones, only doing it for 20-30min means that the enzyme doesn't have enough to work on every long chain carbohydrate.
2) The alpha-amylase enzyme cuts long chains into big chains and creates dexitrins as well as mono, di and tri saccharides (think sucrose, dextrose, maltose, maltase etc...) and even longer chains which generally can not be fermented out. Because the two generally work together you have the alpha chopping up the chains into a variety of lengths and the beta chewing on the chain with whatever length it happens to be. The results of doing a 148 and 162 is that you can a high amount of conversion to the short chains and a high amount of dexitrins added to your beer producing body.
You end up with a moderately fermentable beer (with the max being 148 and the min being 162-165) with excellent body. When I do my decoction mashes, I have to be very careful about resting with a long protein rest for head retention.
Here is the trick though, the only reason to really decoct at this point is to get a slightly higher efficiency but really it's all about the flavor added with the boiling of the grains and husks. Same with a step mash. Right now malsters have done a ton of work to basically get your single infusion mash to be very high efficiency with minimal effort so you should get very similar results to a step mash if you do a single infusion in the middle of the 148-165 range, 155 is a good mix.
Again, this does not work with wheat malt. Doing a single infusion heff requires rice hulls because there is a ton of protein that causes a stuck sparge. Boiling the wheat malt also breaks down a lot of the protein chains. I have done upwards of a 70/30 wheat/barley heff which great results and no rice hulls because I follow a very rigorous mash schedule of 20min at 113, 20min at 122, 30min at 148, 30min at 162, lauder and batch sparge with 168-172.
I have to say, I have never done BIAB, so I am not a great person to ask about that sort of thing but this is the basic chem behind what you are doing. A great book is Noonan: "New guide to brewing lager beer" which dives into brewing chemistry. I would highly recommend picking up these books... they are awesome and well worth the 50$.
