If I may chime in, I love BIAB. I have nailed it down pretty good for what works for me, and get 83% efficiency pretty much everytime. Let me try to help you out a bit based on your statements above.
This recipe used Golden Promise, first batch was primarily US 2 row.
The diastic power difference between these two grains will not really affect your efficiency that much.
I overshot the temp in the first round, nearly 160 degrees for the whole time . This time was right at 150 for about 40 min, then kicked the heat on to 160 for about 15 or 20 min.
Sorry to hear this, but why did you leave it at 160f? I highly highly reccomend adding cold water to your mash next time (little tiny bit at a time) until you hit your target and starting your mash clock at that point. Also: Why the 160F ? Is this a step mash or were you trying to mash out?
I agree - Your temp was a small part of your efficiency, but not a big one. The biggest reason your efficiency was so low in batch one was your mash thickness. My efficiency went past 80% when I started making sure my mash was thin enough to ensure I could stir properly to get all the grain soaked with water. What I like to do is mash in 5 degrees or more too HIGH, then stir like a madman for 5-8 minutes. By then, I am usually at my mash traget temp and I can lid the sucker. If I am too high, I add a tiny bit of cold water, stir, until I hit my target.
Our processes differ a bit though, as I mash in 1 or 2 pots and place them in an oven preheated to 170F (I turn offf the oven once the pots are inside).
I also sparge for 10 mins. I think the sparge helps. All I do is lift my grain bag out of the tun after conversion, and place it in another container (old crappy pot works well). I then move my first runnings to my boil pot, and use put my bag(s) back in the tun(s). I add 175F(ish) water and make sure I am around 165F. I lid for 10 mins, take out the bag, and add the second runnings to my boil pot. I also have a measure stick to make sure my volumes are ok.
So, for you since you have a keggle you may not be able to sparge depending on how many spare pots you have or if you want to bother with all that lifting. 78% is pretty good without a sparge, and it was most certainly that your mash was not as thick so you got better absorbtion into your grains.
Hope that info helps