Whilst the Dreamliner is a better fit for the route, don't be fooled on how a route is performing based on what you can see. Qatar makes a ton of money on the bit you cannot see, the cargo. That is part of the reason an A319LR was a definate no-go for Manchester. (I know range is a slight issue too, but, it was mostly lack of cargo which sealed the decision)

Also, as reported elsewhere, loads on both QR flights have been in in the 80-90% avarage range for 2-3 months now, reflected in the fact the slightly larger A330-300 is used more than the A330-200 lately too.