As a VFF I often get the legroom seats and I know the situation so I stow my bags. However, the concern is not normally that it might be stolen - the problem on most flights these days is that people are often not very gentle when attempting to stuff too many bags into the lockers. The result could be a broken screen on your laptop.
The answer I find is to get onboard early and make sure your bag is stowed in a way that it can't easily be bashed.
By the way, I was pleasantly surprised recently when CX revised their hand baggage policy so that as a Diamond member I can now legitimately carry 4 bags totalling 23kg as cabin baggage (plus overcoat, umbrella & camera), even in economy!