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I've been doing some research into High Speed Sync (aka Auto FP in the Nikon world) and its effects on maximum flash distance.

Essentially, in these High Speed Sync Modes, the Flash is staying "on" (actually, it's just pulsing really fast) for the entire time of the exposure. Despite the fact that with HSS, we're talking about very fast shutter speeds, a usual flash duration can be even faster than that. Sometimes as fast as 1/10,000 second. Because of the way a focal plane shutter works, the faster your sync speed is, the less light from the flash is getting through the camera at any one time. This means that increasing your shutter speed by one stop effectively reduces your maximum flash output by one stop. However, as long as you are working at ranges where you are not required to use maximum flash output, this may not mean there is any change in flash intensity in your situation. With TTL metering, your camera's brains will figure that out for you, and if you're manually metering, you can just increase the flash intensity.

But, it's good to know exactly what effect using HSS will have on your maximum flash distance. Of course every flash will be different depending on its output and reflector/diffuser characteristics. On average, your flash's range will be reduced by about 1/2. However, this is largely dependent on the amount of ambient light you're trying to balance with. I'd like to offer a real world example however, it seems that non of the primary manufacturers are producing spec sheets for HSS Flash. I have been able to find what I think is a spec sheet for the Sigma EF-500 Super DG, so, that's what I'll use.

A quick glance at the specs shows that engaging HSS cuts the 105mm Guide Number from 50 to 25 at 1/250s. Put another way, you lose 1 full stop of power. Or, another way, you cut your maximum flash distance in half. From that point on however, assuming you're adjusting your aperture along with the shutter speed, the maximum distance remains basically constant.  If you'd prefer some real numbers, read on.

The EF-500 Super DG has a guide number of 50m at ISO 100 when zoomed to 105mm. On a bright sunny day, we know that proper exposure would be f/16 for 1/100 second. To exactly match this with our flash, we'd need our subject to be 3.125m away. Adjusting our shutter speed to 1/250 (a common sync speed limit) leaves us with an aperture of f/10 and a distance of 5m. Now, let's say we really want to shoot at f/5.6, proper ambient light exposure would require a shutter speed of 1/800. Here's where things start to change. Our flash has a guide number of 14.0 at 1/800 and zoom at 105mm. At f/5.6 our range is only 2.5 meters, or 1/2 of what it was at 1/250. In other words, if we were using our flash at MAXIMUM power before going into high speed sync, the only way to widen the aperture would be to bring the flash closer to the subject by 1/2 which isn't exactly what we're going for here. To continue the exercise, however, let's say we actually want to shoot at f/4 now. That would be a shutter speed of 1/1600 to match ambient light. Our flash has a guide number of 9.9 at 1/1600 and zoomed to 105mm. This gives us a maximum range of 2.475 meters. Not quite the 2.5m we had before, but not nearly as bad a bite as our first initial hit.

Now let's imagine we aren't working at maximum range to begin with. Let's go back to our original scenario but let's put our subject at a distance of 2 meters. Before engaging HSS, we're shooting at f/10 and 1/250 just like before. At f/10 and our subject at 2 meters our flash is more powerful than we need and we'd have to decrease the power in order to maintain this aperture. Now, to get an aperture of f/5.6 we decrease our shutter speed to 1/800, just like before. Our maximum distance of the flash just like before is 2.5 meters at 1/800. We're still well within range of our subject however, if we are manually adjusting the flash output, we will have to turn it up some since we're closer to our maximum than we were before. Now, taking the final step from before, we're shooting at f/4 and 1/1600. Our maximum distance is 2.475 meters which means, again, we won't have to get any closer to our subject and will simply have to increase the flash power a little if setting it manually. In fact, as long as we intend to keep the same overall ambient exposure, once we take that initial hit, our distance is basically unaffected.

Special thanks to Michael Moore, an incredible wedding photographer from Ontario, Canada, for pointing me in the right direction.