Mr S.,
Be careful with my Delta there - it's a "change all the kinetic energy to potential energy" Delta, so gives the maximum possible height gain, and not a normal small-change small-d delta.
(Delta-v)(Delta-v) = 2g (Delta-h).
As 1kt is very close to about 0.5m/s, for 400kt, Delta-h ~ 200x200/20 ~ 2000m, as HN39 said.
It's for a dragless/liftless object, so more like a bullet than an airliner.
For a normal small change delta-v: v delta-v = g delta-h.
There's also the remaining thrust at work in TWA800, assuming the engines kept running.