Thanks, you're the only reply that understands my need (almost). I do plan
to use a ND filter to reduce DOF.

If I can cheat the sync speed, I could effectively reduce the sunlight's
intensity and enable fill-in flash at greater shooting distance, or save
flash power at the same distance (battery pack last longer), and less
blinding flash into the subjects' eyes.

This auto-limiting sync speed in *manual mode* reminds me of parents who
tell their child they can do whatever they want, and then meddle in the
child's business.

As I'm sure you know, flash sync speed is driven by shutter mechanics.
Faster shutters allow faster sync speeds, the best for FP shutters being
1/300 (Minolta, Nikon).  Canon max sync speeds are 1/250.  (Some Nikon
dslr's like the D70 also have electronic shuttering in the way they use
the sensor thereby achieving 1/500 flash sync).

It's tough doing fast shutters.  Nikon have reduced the max sync in the
F6 to 1/250 from 1/300 (F5).  OTOH, the added value is also questionable.

With my Maxxum 9 (flash sync=1/300) I can get 1/350 shots using the
flash sync terminal.  With my Maxxum 7D (flash sync=1/160) I can get
1/200.  With a Maxxum flash it of course limits it to 1/300 and 1/160
respectively (and 1/125 with anti-shake activated).

With that and your experience it's reasonable to assume that the max
shutter speed is conservative by 1/3 to 2/3 of a stop to allow for
manufacturing variance, environment (temperature) and aging.

Bret and John recommend high-speed sync but that reduces your available
power considerably, esp. when you get to 1/1000 and faster.  Most of the
flash light is wasted on the shutter curtains... it remains a good way
to get shallow DOF on bright scenes where you want some action freeze
and fill in the foreground.

But then so does a ND filter with normal sync flash... and paradoxically
due to the great inefficiency of HSS, you'll actually save flash
batteries with a 2 stop ND in place once you're at 1/1000 (or so) and
faster.

Cheers,
Alan

When I attach a canon speedlite 550ex (high speed sync off) on a 20D, the
shutter speed automatically reduced to 1/250s if I had it faster.

Is there a way to override this?

I found by experiment, I can flash sync at 1/320s (the experiment is done
with a 3rd party flash that does not change the shutter speed). So I would
like to use this speed with the canon speedlite, but it won't let me.

Any ideas? I checked all the custom functions and none of them look like it
would help.