From the B777 FCTM:
"Ideal descent planning is to allow an idle / minimum thrust setting throughout the entire descent sequence and initial stages of flap extension. Progressive flap extension is ideally made when approaching the minimum flap manoeuvre speed. Speed and configuration anticipation will help avoid excessive thrust activity at each intermediate flap manoeuvre speed..."
"...Speed Brakes are the primary means of producing drag, both for profile correction (vertically) and immediate speed reduction.
• Premature flap extension is not to be used to achieve drag. Only in tight / urgent situations combined with full speed brake use, should extension be considered close to flap limit speeds.
• Flap extension is used to facilitate speed reduction either in the normal course of approach speed reduction, or at the request of ATC..."
The Gear Down/F20 technique 2 dots below the GS is a "one-size fits all" profile recommended by Boeing, but it's not always the most efficient way to fly the aircraft. For example, what if you're intercepting the GS at 4,500ft as happens in HKG?
There's nothing wrong, per se, with using a higher flap setting (F15 or 20) if extra drag is required to contain the speed down the GS. However, in that case the preference would be to reduce back to the F15/20 flap manoeuvring speed rather than barreling down the GS at some higher speed. That may not be appropriate if ATC speed control is in force.