4000 lbs is a lot, and will requires 4000 more lbs of lift. This means you need more AoA for that lift, and you get more induced drag. Top speed - at least in the real word - is not just a factor of drag, but of weight.
This whole thread highlights some of the problems of parametric simulation, especially with a paucity of data like military aircraft. I would assume ED has drag index factors for all available stores, but that's mostly an estimation for fuel planning purposes and the rest is going to involve some guestimation.
