Gee...you know we're both writing in English but not communicating. I'll try waving my hands on this one.
First let's define "delay." That is the time from the moment the shutter is released until the flash is triggered. The flash on a leaf shutter is NOT triggered immediately upon shutter release, so by definition there is a delay. OK??
When you release a leaf shutter, there is a gear train connected to a spring drive gear mechanism that provides flash synchronization. This gear train is the "delay mechanism" AND the sync mechanism in the shutter. Gears have to turn a certain distance (delay) before the flash is triggered (trigger mechanism). If you would like confirmation of the sync gear train, please get a service manual for a Compur or like shutter and you will see the part numbers for the flash synchronization gears, etc.
The flash is not triggered by the blades reaching the open position but by the gears and how far they have turned, which MOSTLY corresponds to the position of the blades being fully open - but, does not EXACTLY correspond to the fully open position at ALL shutter speeds. There are inaccuracies inherent within timing of the gear train trigger mechanism versus shutter speed setting.
Hence the information from Stroebel about the flash being measured as 59% less at 1/500th than at 1/100 or less - something YOU seem to want to ignore - actual tested, and documented results by a recognized expert in the field of photography and photo science. Not an opinion, not conjecture, not speculation - documented facts.
As for exposing at 1/60 or less - yes that's still my advice if you are using manually set power levels and NOT an automatic flash with a quench circuit - AND you have a shot that cannot be easily duplicated. When you talk about long flash exposure times only at full power you are both correct and somewhat wrong. If the flash uses a quench circuit then yes, the flash duration gets rapidly shorter than the longest duration at full power because you are using a fully charged capacitor, and stopping the current when the appropriate exposure has been reached.
With this type of flash operation, flash speeds can be as short as 1/10,000 of a second. Most flashes with quench circuits will not go faster than 1/10,000 as when the flash time gets shorter than that, you can have a problem with reciprocity (failure of the law of reciprocity) just like you do with long exposure times.
Now, if you use a flash that allows power variation (1/2, 1/4 power, etc.) the duration is not effected nearly as much as the quench circuit limiting the exposure, because you are only discharging a partially charged capacitor. Using a flash in this mode rarely approaches a flash time of 1/1000 of a second.
My personal flash usage is mainly with large power packs (3000 watt/second ratings) and multiple flash heads in architectural photography. And yes, I have to run nearly everything at full power with multiple heads being used on a lot of occasions where I am not attempting to only provide fill for daylight, but fully illuminating an entire scene.
I rarely use a hand-held unit with thyristor quench circuit. As I stated previously, I have noticed under exposure using 1/250th of a second or higher shutter speeds and my strobes - hence, my comment about "your mileage may vary." Meaning, your personal equipment AND usage may NOT create this problem. Is that clearly understood? This is not a hard and fast edict for everyone. It is a problem that I have had and have documented as being related to the shutter speed. Not and equipment problem, not an exposure miscalculation, not a strobe problem, the shutter speed was too short.
You are correct, not everyone will have this problem, but my point is if you are doing flash photography using leaf shutters - YOU SHOULD KNOW ABOUT THIS AS A POTENTIAL PROBLEM. Your approach seems to indicate that it will NEVER be a problem. This is forum allows educational opportunities - I chose to point out a documented anomoly inherent within leaf shutter / electronic flash usage under certain circumstances.
My advice should have been - test your equipment under a variety of operating conditions, including power levels, and shutter speeds as underexposure is a real potential problem with certain flash / shutter speed combinations. Does that suit you somewhat better?
And, I too am sorry if you don't like my recommendations which are based on both photo science documentation AND personal experience - and MY ONLY POINT BEING (again, apparently ignored by you) - is that:
>>>IF<<< someone has underexposed flash photos
>>>AND<<< they are using a high shutter speed with a leaf shutter,
>>>THEN<<< it may be BECAUSE of the high shutter speed,
AND NOT AN ACCIDENT, OR MISCALCULATED EXPOSURE.
I'm truly sorry IF YOU don't get that simple message.