Dont Hang Up -
I think we have been around this one before. Hazlenuts39 has made a detailed analysis of airspeed in one of the previous threads.
I am mere SLF, but my maths says that kinetic energy would be based on TAS (kinetic energy does not know, or need to know, about air density).
A quick calculation on this basis would have the resulting speeds drop by about 20% for a ~900m climb - or about 260 knots (IAS) down to about 210. I believe this is consistent with the BEA report at the point that speeds returned?