I'm not too fond of brown ales but I really like my Kentucky Commons. Brown ales just seem kinda boring usually. It's probably the chocolate malt and corn that makes it different.
I do sparge, twice actually. I don't add any extra grains after I mash in. I just mash in, cover, walk away and come back 24hrs later. You can always increase the mash time for more sourness. I think 24hrs gives a slight sourness to still be a session beer. I couldn't drink a real sour beer as a session beer.
As for efficiency, I wonder why you didn't get higher. I assume the 24hr mash along with a small grain bill gives such high efficiency. I have got 98% on all mine so far. Another guy got 94% with a slightly different recipe. Not sure what any other people have got. I get pretty good efficiency on other beers, but no more than 85% on regular beers around 1.050 OG.