This video is misleading; there is no "optimum" CG to be determined by experiment. CG location is a compromise between longitudinal stability and trim drag. As the CG moves aft, trim drag and stability decrease until the neutral point where the aircraft is neutrally stable; beyond that the airplane is unstable. The aircraft designer sets the published aft CG limit a bit forward of the neutral point for a safety margin, so there's always some positive stability. The most forward CG is limited by elevator effectiveness, the ability to hold the nose up in a flare.
With power changes, the airplane will always try to maintain its trim speed. Trim for 80 knots at cruise power and chop the power... the nose will drop, the airplane speeds up, and after a few oscillations will settle into an 80 knot glide, more or less. At aft CGs this tendency is reduced, until at the neutral point there will be no nose drop; instead the airspeed will slowly bleed off. The engine thrust line and the trim system design both affect how the plane behaves.
At the max aft CG, trim drag is lowest, the airplane is "livelier" in pitch, pitch stick forces are lower, and the amount of trim necessary to hold speed is less; with a forward CG the handling is more sluggish, the controls are heavier, it requires larger trim adjustments, and performance will suffer a bit.
It's unsurprising that the CG will be well forward with only the front seats occupied and no baggage, as the airplane has to be able to handle a full load without having an excessively aft CG. With a light load, adding weight aft may well improve handling and performance. The allowable aft limit may well be the "optimum" point (and it is, from a performance standpoint, no experiment necessary). Or the pilot may prefer a bit more stability at a small performance cost with the CG a bit farther forward; that you can determine by experiment. But unless approved by the aircraft manufacturer, adding weight behind the designated baggage area can be very dangerous; even though the aircraft may be within the CG limits, the extra weight aft could cause problems with spin recovery.
