I hope this is not a controversial opinion, only my experience and what I learned while shooting film SLR's and getting started ...
In broad daylight, an ND filter not only will serve to keep the shutter speeds to a reasonable level, but also increases contrast in the final image. For this reason, if I do go out shooting in the middle of the day, I will bring an ND filter in my bag for the contrast and not for the shutter speeds. The latter is a byproduct of the former, for me. It's like wearing sunglasses.
The only time recently I've been out and needed to use 1/4000 (or could have needed faster), coincidentally, was when I brought with my on my motorcycle only my Sigma f/1.4 and my XTi, and forgot my filter pouch!
Since its the snapshot thread, I like to add a snapshot with these discussions
Here's the shot from that day ...
Diavel on A1A by
ddk632, on Flickr
So I think the max shutter speed is a really small feature to base this decision on.
Look at it this way: say you but a Mark III - will you, when asked, say that your primary reason for the purchase was the 1/8000 max shutter speed? Or (more likely) you'd justify the decision with AF and FPS?