Setting the preliminary V2 has been a procedure in my airline for 15+ years. You don't have to do a complete performance calculation to get it -- looking at the Vref chart for the proposed takeoff weight is sufficient, or enter the takeoff weight into the Approach page. Can be done in 10-15 seconds. With a TLR (preliminary performance report attached to the dispatch release), just read the V2 off the sheet for less time spent. So don't make rash assumptions about what I am advocating.
And yes, many of the other "inputs critical to correct figures" are made prior to knowledge of the final figures, to include PROPOSED runways, departure, route, altitudes, and arrival. There are procedures and crosschecks for all. You don't wait for your final oceanic clearance before putting the route into the FMS; and when you do get the clearance you crosscheck with your filed route. Same with PROPOSED altitudes and winds to generate a fuel burn schedule...
Finally, we require an entirely new dispatch release if the actual ZFW is more than 10 Tonnes above the original. As I indicated earlier, a 10-Tonne error (in a 747) is survivable due to safety margins if an error is carried forward.
Last edited by Intruder; 17th Jun 2015 at 12:51.