I found some facts about airliners braking performance at http://wiki.flightgear.org/Airliner_brake_performance
Then at Edwards AFB I made tests of brake efficiency from 180 kts to full stop with only brakes, no reversers, no flaps / spoilers / airbrakes :
Concorde 16 secs / 5.8ms-2, 750 meters of braking - unrealistic
A340, B777, A380 omega similar to Concorde - considered as strongly unrealistic
B707 23 secs / 4ms-2, 1100 meters of braking considered as possible
IDG A318 27 secs / 3.4ms-2, 1300 meters of braking - should be OK
IDG A330 31 secs / 3ms-2, 1500 meters of braking - again OK
B747 36 secs / 2.6ms-2, 1700 meters of braking - considered as realistic
Repair of the unrealistic aircraft is simple, as Thorsten wrote in his post :
For JSBSim at least, the drag / friction coefficients in the FDM definitions should do the trick. Reversers might depend on the implementation, whether they're hacky or not.
My Concorde has been fixed - now is 30 secs / 3ms-2 and around 1450 meters from 180 kts to full stop with gross weight 130 tons. With that parameters and maximum landing weight 111 tons is still possible land at Paro, but it is very difficulty. Landing at Merida (SVMD, Venezuela) is almost attempt for suicide.