I have the WTB Nanos in 27.5x2.1 on another bike (and have had the 29x2.1 before) and they ride pretty nicely and have tremendous grip off the road. But you definitely do get tread squirm and some rumble from them on pavement, which reduces their efficiency. I think they're a great 50/50 on/off road tire, depending on how "off the road" your 50% off road gets.
What is the size of your 40s,
rumrunn6 ? Are they a true 40mm? I do like how tall they usually are...many tires form sort of a squished "C" in cross section, but Nanos are usually more like a tall "U" when inflated.