I can do that with my 4wt.
I actually have been experimenting quite a bit with single handed spey technique this summer with mixed results. What I have found is that, in the right stream, I can add about 15 feet to the end of my roll cast, and if I employ the fighting butt of my 8 wt as a half-assed handle I can make that almost 25 feet. That said, it is clear that you really do need the extra length of a Spey rod to really make it look like it supposed to. I think it is just difficult to pull off any of the "advanced" techniques when you are manipulating the line just 7 or 8 feet above the surface of the water.
I just bought a Royal Wulff Triangle Taper (
Edit, it is the "Ambush" Triangle Taper line) shooting head line, and I am looking forward to getting it out on the river this week.