Hmm...that's tough. 40% into an attenuation is really early, which makes me suspect (like you) that it's a conversion problem. Can you grab a bottle of iodine to do some starch tests? I'd be surprised that simply switching to BIAB is killing your efficiency so dramatically, unless you've got a really course grind and the extra time sparging was helping before. That said, I can't really see what else it would be.
As for the sugar, seems reasonable enough. If you do have unconverted starch, though, it will only go so far to help. The lambic bugs won't eat the starch, but they can help to make an otherwise underdeveloped beer a bit more complex. Good luck