He has some great videos. I watched this one and was impressed at the calculations, the transparency and the celebration at the end when he reached supersonic speeds.
I suspect that with a little iterative modelling of the whole system with derivatives, efficiency could be bought from ~40% to ~80%, giving you an extra few hundred mph...
I would think you want it decreasing to near zero to extract all the kinetic energy from the mass, leaving the mass stationary as it hits the ground.
He has some great videos. I watched this one and was impressed at the calculations, the transparency and the celebration at the end when he reached supersonic speeds.