I don't exactly know what you mean by "down size", but I wear between 30W and 31W. 31W Thanaz is almost too big for me, but I went ahead and ordered 31W Thavar anyways and they seem to fit okay.
If you have seen my Thanaz fits, you can tell that these are definitely looser, but I actually like having a little breathing room for my thighs :) I don't see why 29s wouldn't fit you, but then it just depends on how you like wearing your jeans, of course.
EDIT: I have heard 8NE stretches -alot- though, but these are fairly new to me so I haven't experienced it yet :S so maybe I am not the best person to ask.