The BRS master plan is still the one published in 2006 in response to the then Labour government's earlier white paper. Many things were not anticipated in the master plan (and in fairness could not reasonably be expected to have been), not least the dreadful recession that visited the world in the next few years.
To that extent then the master plan cannot be relied on completely as a guide to what might now transpire, although the period 2016-2030 was only ever set out in outline form.
The 'forecast' for the period up to 2015 has turned out to be somewhat optimistic with, for example, a projected 8.1 mppa at the end of this year (around 6.6/6.7 mppa will probably be the actual total). BRS is not alone in this: neighbouring airports at CWL and EXT suggested over 4 mppa and 1.5/2 mppa respectively for 2015 in their master plan projections which were prepared at approximately the same time as BRS's.
As bristolflyer mentions, the airport put its hopes in the new generation of aircraft such as the B 787 and the A 350 as a means of overcoming some of the limitations of its runway. Whether that will turn out to be justified remains to be seen. After withdrawing the B 767 on the summer routes to Sanford and Cancun two years ago there was disappointment that TUI did not operate the B 787 on transatlantic charters, especially as MD Chris Browne said publicly in 2010 that BRS would be amongst the first UK airports to operate the Dreamliner. Whether this is a commercial or operational decision is not known.
In its master plan the airport said that passenger forecasts (then) showed there was limited demand for long haul scheduled routes and the airport identified four: New York (then being operated - to EWR), Washington, Dubai and another US destination that it suggested might be Atlanta. However, it did see further demand for long haul charter services.
Apart from the size of its site, BRS long-haul scheduled potential does suffer from its proximity to LHR which is the default choice for many local business people, and with electrification of Brunels' main line plus a spur into LHR access will be easier from the Bristol area in the forseeable future. One of the reasons cited by Continental for its axing its BRS-EWR flight was the poor take-up in business-first. Overall passenger numbers met and exceeded the airline's publicly stated target in the beginning, although the recession undoubtedly took its toll later on.
I wasn't aware of Continental's B 757-200 being weight-restricted by passenger numbers at BRS. It began as a 172-seat aircraft and later increased to 175 seats. BRS sees very little cargo to/from anywhere according to CAA stats.