Nothing in the rules about ditching parts....
Max speed on this run was 432 m/s
I did get a run of 438, but I did go off the runway and it was in a slightly different vehicle
Edit: Redid with more courage, got a run of 447 m/s
Stupidly designed, has way too much liquid fuel. Gets into orbit in less than 5 minutes, courtesy of 2 rapiers. Apparently, once you get going fast enough, a single ram air intake is sufficient for both rapiers.
Being heavy helps. Try one of the smaller docking ports with like a tiny probe. I just tested a stayputnik on some fuel tanks on the runway and with gravity hacked, it drifted at about 0.2 m/s
As I said, try pointing down a bit while burning at apoapsis. Do you have the tracking station upgraded? Then you can create a maneuver node by clicking on the orbit.