I tested this further and it looks like Dead Ball+/Architect may be applying a hidden cap or normalization to free-kick ball speed rather than scaling linearly with Shot Power from 90 to 99
EA describes Dead Ball+ as giving set pieces “max speed, curve, accuracy, and preview line,” so if Dead Ball+ already pushes the free-kick mechanic to its maximum effective speed, then extra Shot Power may not increase the displayed km/h value.
That would explain why shots stay around 158–168 km/h even with 99 Shot Power.
However, it would still be useful if EA clarified whether 168 km/h is an intended cap for Pro Clubs free kicks, or whether the Architect specialization/Dead Ball+ is not applying correctly. Right now the in-game wording makes players expect a difference between 90 and 99 Shot Power, but testing suggests there may be no practical speed gain.