Mark, I don't know if you have followed my posts about converting Arion to junk sloop from bermudian cutter.  I was a bit intimidated by how different it all was at first but reminded myself that a legion of veteran sailors had successfully voyaged with the rig before me.  My experience with the rig has been that my cambered sail works just as well to windward as the bermudian rig did, more so in fact because the boat stands up to its canvas so much better than before, allowing me to drive the boat a lot harder.  I always put a tie around the bundle when deep reefed.  As David says, it helps with sail shape but it also guards against fan ups when gybing.  With my sail deeply reefed, the rig feels rock solid and inspires tremendous confidence.  Knowing how easy it is to raise or lower a panel as needed only adds to my sense of snug security.   Having experienced cruising under junk rig, I would never go back to a bermudian rig.

A HM sail reefed down to the top two triangles is actually quite an efficient shape, and is very manageable when going to windward in half a gale. It's better if a sail tie is put around the whole bundle of sheetlets, so that they all pull together, and don't let the sail sag off, but that's easy to do. 

The standard junk sail plan gives a very low aspect sail when reefed down to its smallest size.  In theory this should not perform as well as a higher aspect sail up wind.