I have a di2 Cannondale road bike that I barely use. I know it's not an MTB but it is pretty stock aside from FSA k force light cranks over the si ones it came with. It is very, very light, but I assumed nothing out of the ordinary. I took it to my lbs as the di2 kept losing charge quickly and wanted it sorted. They weren't equipped to do anything, but they did comment that it was so light I wouldn't be able to race it in some categories. I had no idea what this meant and have no intention of racing it ever, but just another example of a very light Cannondale (alloy frame, caad 10 black Inc).
I've no other light stock bikes but my dynatech tours is very light for an MTB, even with steel forks (though triple butted).