We all use simulators on a regular basis but still depend on calculation results done by our performance specialists. How exactly he calculated is at least hard to find out if not impossible.
How about this:
They give us their performance program, runway and obstacle data and we complete those with contamination data and run the program ourselves...
Here is another variable I would like to hear more of: On long enough runways - and here contaminants impeding acceleration are taken into account - a big Swiss Airline is using 1.4 times V stall on the Airbus family for a given weight because this shall give better climb angles than 1.2 times V stall and thus higher obstacle weights.
Only with braking impeding conditions as bad friction coefficient or allways with water or slush deeper than one mm - considered to result in aquaplanning - they are willing to reduce V2 to "normal value".
They swear to do a realistic calculation for every runway condition but still I would like to enter the variables myself just to see what happens on "my performance simulator"...